External cathode ion source

ABSTRACT

An ion source is disclosed for use in fabrication of semiconductors. The ion source includes an electron emitter that includes a cathode mounted external to the ionization chamber for use in fabrication of semiconductors. In accordance with an important aspect of the invention, the electron emitter is employed without a corresponding anode or electron optics. As such, the distance between the cathode and the ionization chamber can be shortened to enable the ion source to be operated in an arc discharge mode or generate a plasma. Alternatively, the ion source can be operated in a dual mode with a single electron emitter by selectively varying the distance between the cathode and the ionization chamber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/059,608 filed Mar. 31, 2008, which is a continuation-in-part of U.S.patent application Ser. No. 11/648,506, filed Dec. 29, 2006, which is acontinuation of U.S. patent application Ser. No. 10/887,426, filed onJul. 8, 2004, now U.S. Pat. No. 7,185,602, which is a divisional of U.S.patent application Ser. No. 10/170,512, filed Jun. 12, 2002, now U.S.Pat. No. 7,107,929, which is a continuation of International PatentApplication No. PCT/US00/33786, filed Dec. 13, 2000, which claimspriority of and benefit of U.S. Provisional Patent Application No.60/170,473 filed on Dec. 13, 1999, U.S. Provisional Patent ApplicationNo. 60/250,080 filed on Nov. 3, 2000.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to an ion source for use in semiconductorfabrication and more particularly to an ion source configured with itscathode located external to the ionization chamber, the ion source beingoperable in a first mode of operation or may be configured as a dualmode ion source that is selectively operable in both a first mode ofoperation, such as an arc discharge mode and a second mode of operation,such as a direct electron impact mode of operation, with a singleelectron emitter that can be used for ionizing gases and vapors andproducing monatomic ions in an arc discharge mode of operation andmolecular ions, such as cluster ions in a direct electron impact mode ofoperation.

2. Description of the Prior Art

Electron emitters (also known as electron guns) with both directlyheated and indirectly heated cathodes (IHC) are known in the art to beused in ion implantation systems. In known ion implantation systems,electron emitters with such heated and indirectly heated cathodes arenormally disposed within an ionization chamber and are therefore subjectto a relatively harsh environment as a result of the plasma developedwithin the ionization chamber. Electron emitters with IHCs offeradvantages over those with directly heated cathodes in reliability andlife time. In particular, emitters with directly heated cathodes includea relatively small wire that forms a filament. These filaments are knownto fail in such harsh environments in a relatively short time. On theother hand, electron emitters which include Indirectly heated cathodesinclude a relatively massive cathode that is heated indirectly byelectron bombardment from a filament. In the case of the IHC electronemitters, the cathode emits electrons thermionically. Although, the IHCis exposed to a harsh plasma environment, the massive cathode having asubstantially larger mass than the filament of a directly heated cathodeprovides a relatively longer life than such directly heated cathodes.

As such, IHC electron emitters are typically used to ionize feedmaterials in hot arc discharge mode, where the IHC emitter sits insidethe ionization chamber. The filament inside the IHC electron emitter isheated with electrical current, and biased into negative potential withrespect to the solid emitter block. This allows electrons from thefilament to be accelerated into the solid emitter heating it up. Theemitter block in turn is biased negatively with respect to the ionsource body. When the emitter reaches sufficient temperature, it willstart emitting electrons igniting arc discharge between the emitter andthe source wall forming plasma.

In order to further increase the operating lifetime of such IHCs,devices have been developed which include a cathode and a filament inwhich the cathode is located inside the ionization or arc chamber of theion implantation system while the filament is located outside theionization chamber, as described in detail in Varian U.S. Pat. No.7,138,768. With such a configuration, by locating the filament outsidethe harsh environment of the ionization chamber, the operating lifetimeof such IHCs is relatively longer than IHCs which are totally locatedwithin the ionization chamber.

In recent development of cluster ion sources, heat management has becomeincreasingly important, as many of the cluster molecules are quitefragile and can be dissociated when exposed to hot surfaces like the IHCemitter. It is thus beneficial to remove the electron emitter form thesource and situate it externally, namely, outside the ionizationchamber, for example, as disclosed in SemEquip U.S. Pat. No. 7,185,602,hereby incorporated by reference. Also the significant material depositsthat some of the cluster materials produce favor the removal of theemitter from the source, as there is a risk of building up flakes thatwill short the IHC to the ionization chamber potential.

This will change the operational parameters of the emitter somewhat. Nowthe emitter is removed from the higher gas pressure of the ionizationchamber into the usually pumped source housing, where the gas pressurecan be an order of magnitude or more lower, striking an arc dischargebetween the IHC emitter and the source will become more difficult. Ifplasma is ignited, there will be significant diffusion losses before thebulk of the plasma would get into the ionization chamber. For clusterionization, the required extracted beam current densities are typicallyan order of magnitude lower than for traditional atomic implant species.This means that dense plasma is not needed in order to create sufficiention beam currents. Externally located electron gun which forms anelectron beam either from a directly or indirectly heated emitter willbe able to inject sufficient electron current into the ionizationchamber.

To form an electron beam that can be efficiently injected into thesource, i.e, ionization chamber, electron optics are normally used topull and focus the beam. Typically this means placing an anode electrodebetween the source and emitter. The electrons are pulled from theemitter with the assistance of the anode potential and accelerated toenergy e*Vcathode+e*Vanode going across the emitter-anode gap andthrough the anode. At the anode-source gap the electron beam isdecelerated to e*Vcath and focused by the decel lens effect. This setupwill allow for refined tuning of the electron beam current, size andemittance. The downside is that the distance the electrons are travelingwill increase by the extent of the anode and anode gap. Typically theanode voltages are higher than the cathode voltage. This can lead intovoltage holding issues.

Ion beams are produced from ions extracted from an ion source. An ionsource typically employs an ionization chamber connected to a highvoltage power supply. The ionization chamber is associated with a sourceof ionizing energy, such as an arc discharge, energetic electrons froman electron-emitting cathode, or a radio frequency or microwave antenna,for example. A source of desired ion species is introduced into theionization chamber as a feed material in gaseous or vaporized form whereit is exposed to the ionizing energy. Extraction of resultant ions fromthe chamber through an extraction aperture is based on the electriccharge of the ions. An extraction electrode is situated outside of theionization chamber, aligned with the extraction aperture, and at avoltage below that of the ionization chamber. The electrode draws theions out, typically forming an ion beam. Depending upon desired use, thebeam of ions may be mass analyzed for establishing mass and energypurity, accelerated, focused and subjected to scanning forces. The beamis then transported to its point of use, for example into a processingchamber. As the result of the precise energy qualities of the ion beam,its ions may be implanted with high accuracy at desired depth intosemiconductor substrates.

The Ion Implantation Process

The conventional method of introducing a dopant element into asemiconductor wafer is by introduction of a controlled energy ion beamfor ion implantation. This introduces desired impurity species into thematerial of the semiconductor substrate to form doped (or “impurity”)regions at desired depth. The impurity elements are selected to bondwith the semiconductor material to create electrical carriers, thusaltering the electrical conductivity of the semiconductor material. Theelectrical carriers can either be electrons (generated by N-typedopants) or “holes” (i.e., the absence of an electron), generated byP-type dopants. The concentration of dopant impurities so introduceddetermines the electrical conductivity of the doped region. Many such N-and P-type impurity regions must be created to form transistorstructures, isolation structures and other such electronic structures,which collectively function as a semiconductor device.

To produce an ion beam for ion implantation, a gas or vapor feedmaterial is selected to contain the desired dopant element. The gas orvapor is introduced into the evacuated high voltage ionization chamberwhile energy is introduced to ionize it. This creates ions which containthe dopant element (for example, in silicon the elements As, P, and Sbare donors or N-type dopants, while B and In are acceptors or P-typedopants). An accelerating electric field is provided by the extractionelectrode to extract and accelerate the typically positively chargedions out of the ionization chamber, creating the desired ion beam. Whenhigh purity is required, the beam is transported through mass analysisto select the species to be implanted, as is known in the art. The ionbeam is ultimately transported to a processing chamber for implantationinto the semiconductor wafer.

Similar technology is used in the fabrication of flat-panel displays(FPD's) which incorporate on-substrate driver circuitry to operate thethin-film transistors which populate the displays. The substrate in thiscase is a transparent panel such as glass to which a semiconductor layerhas been applied. Ion sources used in the manufacturing of FPD's aretypically physically large, to create large-area ion beams of boron,phosphorus and arsenic-containing materials, for example, which aredirected into a chamber containing the substrate to be implanted. MostFPD implanters do not mass-analyze the ion beam prior to its reachingthe substrate.

Many ion sources used in ion implanters for device wafer manufacturingare “hot” sources, that is, they operate by sustaining an arc dischargeand generating a dense plasma; the ionization chamber of such a “hot”source can reach an operating temperature of 800 C or higher, in manycases substantially reducing the accumulation of solid deposits. Inaddition, the use of BF.sub.3 in such sources to generateboron-containing ion beams further reduces deposits, since in thegeneration of a BF.sub.3 plasma, copious amounts of fluorine ions aregenerated; fluorine can etch the walls of the ion source, and inparticular, recover deposited boron through the chemical production ofgaseous BF.sub.3. With other feed materials, however, detrimentaldeposits have formed in hot ion sources. Examples include antimony (Sb)metal, and solid indium (In), the ions of which are used for dopingsilicon substrates.

A typical commercial ion implanter is shown in schematic in FIG. 1. Theion beam I is shows propagating from the ion source 42 through atransport (i.e. “analyzer”) magnet 43, where it is separated along thedispersive (lateral) plane according to the mass-to-charge ratio of theions. A portion of the beam is focused by the magnet 43 onto a massresolving aperture 44. The aperture size (lateral dimension) determineswhich mass-to-charge ratio ion passes downstream, to ultimately impactthe target wafer 55, which typically may be mounted on a spinning disk45. The smaller the mass resolving aperture 44, the higher the resolvingpower R of the implanter, where R=M/.DELTA.M (M being the nominalmass-to-charge ratio of the ion and .DELTA.M being the range ofmass-to-charge ratios passed by the aperture 44). The beam currentpassing aperture 44 can be monitored by a moveable Faraday detector 46,whereas a portion of the beam current reaching the wafer position can bemonitored by a second Faraday detector 47 located behind the disk 45.The ion source 42 is biased to high voltage and receives gasdistribution and power through feedthroughs 48. The source housing 49 iskept at high vacuum by source pump 50, while the downstream portion ofthe implanter is likewise kept at high vacuum by chamber pump 51. Theion source 42 is electrically isolated from the source housing 49 bydielectric bushing 52. The ion beam is extracted from the ion source 42and accelerated by an extraction electrode 53. In the simplest case(where the source housing 49, implanter magnet 43, and disk 45 aremaintained at ground potential), the final electrode of the extractionelectrode 53 is at ground potential and the ion source is floated to apositive voltage V.sub.a, where the beam energy E=qV.sub.a and q is theelectric charge per ion. In this case, the ion beam impacts the wafer 55with ion energy E. In other implanters, as in serial implanters, the ionbeam is scanned across a wafer by an electrostatic or electromagneticscanner, with either a mechanical scan system to move the wafer oranother such electrostatic or electromagnetic scanner being employed toaccomplish scanning in the orthogonal direction.

As shown, in FIG. 2, a Bernas ion source a is mounted to the vacuumsystem of the ion implanter through a mounting flange b which alsoaccommodates vacuum feedthroughs for cooling water, thermocouples, feedmaterial as a dopant gas feed, N₂ cooling gas, and power. The gas feed cfeeds gas into the arc chamber d in which the gas is ionized. Alsoprovided are dual vaporizer ovens e, f in which solid feed materialssuch as As, Sb₂O₃, and P may be vaporized. The ovens, gas feed, andcooling lines are contained within a cooled machined aluminum block g.The water cooling is required to limit the temperature excursion of thealuminum block g while the vaporizers, which operate between 100 C. and800 C., are active, and also to counteract radiative heating by the arcchamber d when the source is active. The arc chamber d is mounted to thealuminum block g.

The gas introduced to arc chamber d is ionized through electron impactwith the electron current, or arc, discharged between the cathode h andthe arc chamber d. To increase ionization efficiency, a uniform magneticfield i is established along the axis joining the cathode h and ananticathode j by external Helmholz coils, to provide confinement of thearc electrons. An anticathode j (located within the arc chamber d but atthe end opposite the cathode h) is typically held at the same electricpotential as the cathode h, and serves to reflect the arc electronsconfined by the magnetic field i back toward the cathode h and backagain repeatedly. The trajectory of the thus-confined electrons ishelical, resulting in a cylindrical plasma column between the cathode hand anticathode j. The plasma density within the plasma column istypically high, on the order of 10¹² per cubic centimeter; this enablesfurther ionizations of the neutral and ionized components within theplasma column by charge-exchange interactions, and also allows for theproduction of a high current density of extracted ions. The ion source ais held at a potential above ground (i.e., the silicon wafer potential)equal to the accelerating voltage V_(a) of the ion implanter: the energyof the ions E as they impact the wafer substrate is given by E=qV_(a),where q is the electric charge per ion.

The cathode h is typically a hot filament or indirectly-heated cathode,which thermionically emits electrons when heated by an external powersupply. It and the anticathode are typically held at a voltage V_(c)between 60V and 150V below the potential of the ion source V_(a). Highdischarge currents D can be obtained by this approach, up to 7 A. Oncean arc discharge plasma is initiated, the plasma develops a sheathadjacent to the surface of the cathode h (since the cathode h isimmersed within the arc chamber and is thus in contact with theresulting plasma). This sheath provides a high electric field toefficiently extract the thermionic electron current for the arc; highdischarge currents can be obtained by this method.

If the solid source vaporizer ovens e or f are used, the vaporizedmaterial feeds into the arc chamber d through vaporizer feeds k and l,and into plenums m and n. The plenums serve to diffuse the vaporizedmaterial into the arc chamber d, and are at about the same temperatureas the arc chamber d. In this case a co-gas could be introduced eithervia tube c into chamber d if the co-gas was from a gaseous stock, Itwould also be possible to utilize whichever solid vaporizer (e or f) wasnot in use for, the primary feedstock to generate a co-gas from anappropriate solid material.

Cold ion sources, for example the RF bucket-type ion source which usesan immersed RF antenna to excite the source plasma (see, for example,Leung et al., U.S. Pat. No. 6,094,012, herein incorporated byreference), are used in applications where either the design of the ionsource includes permanent magnets which must be kept below their Curietemperature, or the ion source is designed to use thermally-sensitivefeed materials which break down if exposed to hot surfaces, or whereboth of these conditions exist. Cold ion sources suffer more from thedeposition of feed materials than do hot sources. The use of halogenatedfeed materials for producing dopants may help reduce deposits to someextent, however, in certain cases, non-halogen feed materials such ashydrides are preferred over halogenated compounds. For non-halogenapplications, ion source feed materials such as gaseous B.sub.2H.sub.6,AsH.sub.3, and PH.sub.3 are used. In some cases, elemental As and P areused, in vaporized form. The use of these gases and vapors in cold ionsources has resulted in significant materials deposition and hasrequired the ion source to be removed and cleaned, sometimes frequently.Cold ion sources which use B.sub.2H.sub.6 and PH.sub.3 are in common usetoday in FPD implantation tools. These ion sources suffer fromcross-contamination (between N- and P-type dopants) and also fromparticle formation due to the presence of deposits. When transported tothe substrate, particles negatively impact yield. Cross-contaminationeffects have historically forced FPD manufacturers to use dedicated ionimplanters, one for N-type ions, and one for P-type ions, which hasseverely affected cost of ownership.

Recently, cluster implantation ion sources have been introduced into theequipment market (see for example, U.S. Pat. Nos. 6,107,634; 6,288,403;and 6,958,481). U.S. Pat. Nos. 6,452,338; 6,686,595; and. 6,744,214,hereby incorporated by reference, disclose ion sources that are unlikethe Bernas-style sources in that they have been designed to produce“clusters”, or conglomerates of dopant atoms in molecular form,including ions of the form As_(n) ⁺, P_(n) ⁺, or B_(n)H_(m) ⁺, where nand m are integers, and 2≦n≦18. Such ionized clusters can be implantedmuch closer to the surface of the silicon substrate and at higher dosesrelative to their monomer (n=1) counterparts, and are therefore of greatinterest for forming ultra-shallow p-n transistor junctions, for examplein transistor devices of the 65 nm, 45 nm, or 32 nm generations. Thesecluster sources preserve the parent molecules of the feed gases andvapors introduced into the ion source. The most successful of these haveused electron-impact ionization, and do not produce dense plasmas, butrather generate low ion densities at least 100 times smaller thanproduced by conventional Bernas sources, for example, as disclosed inthe cluster ion sources mentioned above The use of B₁₈H₂₂ as an implantmaterial for ion implantation of B₁₈H_(x) ⁺ in making PMOS devices isdisclosed in Horsky et al. U.S. Patent Application Publication No. US2004/0002202 A1, hereby incorporated by reference.

FIG. 3 shows in schematic a cluster ion source 1 as described in moredetail in U.S. Pat. No. 6,452,338, hereby incorporated by reference. Thevaporizer 2 is attached to the vaporizer valve 3 through an annularmetal gasket 4. The vaporizer valve 3 is likewise attached to theionization chamber 5 by a second annular metal gasket 6. This ensuresgood thermal conduction between the vaporizer, vaporizer valve, andionization chamber 5 through intimate contact via thermally conductiveelements. A mounting flange 7 attached to the ionization chamber 5allows mounting of the ion source 1 to the vacuum housing of an ionimplanter, and contains electrical feedthroughs (not shown) to power theion source, and water cooling feedthroughs 8, 9 to cool the ion source.The water feedthroughs 8, 9 circulate water through the source shield 10to cool the source shield 10 and cool the attached components, the beamdump 11 and electron gun 12 (further described below). The exit aperture13 is mounted to the ionization chamber 5 face by metal screws (notshown). Thermal conduction of the exit aperture 13 to the ionizationchamber 5 is aided by an annular seal 14 which can be made from metal ora thermally conductive polymer.

When the vaporizer valve 3 is in the open position, vaporized gases fromthe vaporizer 2 can flow through the vaporizer valve 3 to inlet channel15 into the open volume of the ionization chamber 5. These gases areionized by interaction with the electron beam transported from theelectron gun 12 to the beam dump 11. The ions can then exit the ionsource from the exit aperture 13, where they are collected andtransported by the ion optics of the ion implanter.

The vaporizer 2 is made of machined aluminum, and houses a water bath 17which surrounds a crucible 18, wherein resides solid feed materials 19.The water bath 17 is heated by a resistive heater plate 20 and cooled bya heat exchanger coil 21 to keep the water bath at the desiredtemperature. The heat exchanger coil 21 is cooled by de-ionized waterprovided by water inlet 22 and water outlet 23. Although the temperaturedifference between the heating and cooling elements provides convectivemixing of the water, a magnetic paddle stirrer 24 continuously stirs thewater bath 17 while the vaporizer is in operation. A thermocouple 25continually monitors the temperature of the crucible 18 to providetemperature feedback for a PID vaporizer temperature controller (notshown). The ionization chamber 5 is made of aluminum, graphite, ormolybdenum, and operates near the temperature of the vaporizer 2 throughthermal conduction. In addition to low-temperature vaporized solids, theion source can receive gases through gas feed 26, which feeds directlyinto the open volume of the ionization chamber 16 by an inlet channel27. In prior art systems, when the gas feed 26 is used to input feedgases, the vaporizer valve 3 is closed, however, in an embodiment of theinvention, the feed material is vaporized in the vaporizer 2 andprovided to the chamber 5 as a gas and the co-material gas is alsoprovided to the chamber 5 via gas feed 26. The co-gas is introduced andmetered via a commercial mass-flow-controller.

FIG. 4 illustrates the geometry of the ion source with the exit apertureremoved; the ion beam axis points out of the plane of the paper. Anelectron beam 32 is emitted from the cathode 33 and focused by theelectron optics 34 to form a wide beam. The electron beam may beasymmetric, in that it is wider perpendicular to the ion beam axis thanit is along that axis. The distribution of ions created by neutral gasinteraction with the electron beam roughly corresponds to the profile ofthe electron beam. Since the exit aperture 13 is a wide, rectangularaperture, the distribution of ions created adjacent to the aperture 13should be uniform. Also, in the ionization of decaborane and other largemolecules, it is important to maintain a low plasma density in the ionsource This limits the charge-exchange interactions between the ionswhich can cause loss of the ions of interest. Since the ions aregenerated in a widely distributed electron beam, this will reduce thelocal plasma density relative to other conventional ion sources known inthe art. The electron beam passes through an entrance channel 35 in theionization chamber and interacts with the neutral gas within the openvolume 16. It then passes through an exit channel 36 in the ionizationchamber and is intercepted by the beam dump 11, which is mounted ontothe water-cooled source shield 10. Since the heat load generated by thehot cathode 33 and the heat load generated by impact of the electronbeam 32 with the beam dump 11 is substantial, these elements, as well asthe electron optics or anodes 34, are kept outside of the ionizationchamber open volume 16 where they cannot cause dissociation of theneutral gas molecules and ions.

Borohydrides

Borohydride materials such as B.sub.10H.sub.14 (decaborane) andB.sub.18H.sub.22 (octadecaborane) under the right conditions form theions B.sub.10H.sub.x.sup.+, B.sub.10H.sub.x.sup.−,B.sub.18H.sub.x.sup.+, and B.sub.18H.sub.x.sup.−. When implanted, theseions enable very shallow, high dose P-type implants for shallow junctionformation in CMOS manufacturing. Since these materials are solid at roomtemperature, they must be vaporized and the vapor introduced into theion source for ionization. They are low-temperature materials (e.g.,decaborane melts at 100 C, and has a vapor pressure of approximately 0.2Torr at room temperature; also, decaborane dissociates above 350 C), andhence must be used in a cold ion source. They are fragile moleculeswhich are easily dissociated, for example, in hot plasma sources.

Contamination Issues of Borohydrides

Boron hydrides such as decaborane and octadecaborane present a severedeposition problem when used to produce ion beams, due to theirpropensity for readily dissociating within the ion source. Use of thesematerials in Bernas-style arc discharge ion sources and also inelectron-impact (“soft”) ionization sources, have confirmed thatboron-containing deposits accumulate within the ion sources at asubstantial rate. Indeed, up to half of the borohydride vapor introducedinto the source may stay in the ion source as dissociated, condensedmaterial. Eventually, depending on the design of the ion source, thebuildup of condensed material interferes with the operation of thesource and necessitates removal and cleaning of the ion source.

Contamination of the extraction electrode has also been a problem whenusing these materials. Both direct ion beam strike and condensed vaporcan form layers that degrade operation of the ion beam formation optics,since these boron-containing layers appear to be electricallyinsulating. Once an electrically insulating layer is deposited, itaccumulates electrical charge and creates vacuum discharges, orso-called “glitches”, upon breakdown. Such instabilities affect theprecision quality of the ion beam and can contribute to the creation ofcontaminating particles.

It is desirable at times to be able to run an ion implantation systemfor implanting either monatomic ions or molecular ions, such as clusterions. U.S. Pat. No. 7,107,929 and US Patent Application Publication No.2007/0108394 A1, assigned to the same assignee as the present invention,are examples of ion sources that are configured to operate in dual modesand generate monatomic ions in an arc discharge mode of operation andmolecular ions, such as cluster ions in a direct electron impact mode ofoperation. Some known dual mode ion sources are known utilize a firstelectron emitter which includes a cathode and an associated anode,remote from the ionization chamber which are configured to operate in adirect electron impact mode for the cluster ion feed material and alsoemploy a second electron emitter disposed within the ionization chamber,to ionize the monatomic ion feed material. These electron emitterseither use small gaps around the cathode (with remote supportinsulators) or adjacent insulators to prevent the cathode from shortingout to the ionization chamber. While such dual mode ion sources are asignificant improvement over single mode ion sources, the need formultiple electron emitters in a single ion source adds a substantialamount of complexity.

Thus, there is a need to provide an ion implantation system whichincludes an ion source which includes a single electron emitter, such asan electron emitter that includes a cathode that is located external tothe ionization chamber which eliminates the need for an associated anodeand which can be used in an arc discharge mode or alternativelyincorporated into a dual mode ion source operable in a direct electronimpact mode and an arc discharge mode.

SUMMARY OF THE INVENTION

Briefly, the present invention relates to an ion source with an electronemitter that includes a cathode mounted external to the ionizationchamber for use in fabrication of semiconductors. In accordance with animportant aspect of the invention, the electron emitter is employedwithout a corresponding anode or electron optics. As such, the distancebetween the cathode and the ionization chamber can be shortened toenable the ion source to be operated in an arc discharge mode andgenerate a plasma. Alternatively, the ion source can be operated in adual mode with a single electron emitter by selectively varying thedistance between the cathode and the ionization chamber.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a diagrammatic view of a prior art ion implanter;

FIG. 2 is a diagrammatic cross-sectional view of a Bernas arc dischargeion source, illustrative of the ion source for which the implanter ofFIG. 1 was designed;

FIG. 3 is a longitudinal cross-sectional view of an embodiment of theion source of the present invention with associated vaporizer;

FIG. 3A is a cross-sectional view, similar to a part of FIG. 3, showinganother embodiment of a vaporizer;

FIG. 3B illustrates the removable feature of the vaporizer of FIG. 3A,using a conventional mounting flange while FIG. 3C illustrates detachingthe vaporizer and valve from the ion source;

FIG. 3D illustrates a two-valve embodiment in which separation of thevaporizer from the ion source can occur between the two valves;

FIG. 3E illustrates a dual vaporizer embodiment;

FIG. 3F shows another embodiment of a vaporizer similar to FIG. 3A, butwith a separate crucible and with gas-mediated conduction betweenvaporizer housing and crucible, and between a heat exchanger and thehousing.

FIG. 4 is a side cross-sectional view taken on line 4-4 of FIG. 3 whileFIG. 4A is a top view taken on line 4A-4A of FIG. 4, illustrating aknown electron emitter which includes a corresponding anode and electronoptics;

FIGS. 4B and 4C are views similar to that of FIG. 4, of otherarrangements of the discretely defined electric beam dump;

FIGS. 4D and 4E, side and top views similar respectively to FIGS. 4 and4A, show a conductively cooled ionization chamber assembly having adisposable inner ionization chamber.

FIG. 4F is a three dimensional representation of a broad, collimatedelectron beam and its relation to the ion extraction aperture of theembodiment of FIGS. 3 and 4;

FIG. 5 is a view similar to FIG. 4F of the relationship of a broadelectron beam and ion extraction aperture of narrower dimension;

FIG. 6 is a front view of the aperture plate of the ion source of FIG.3;

FIG. 7 is an illustration of an indirectly heated cathode arrangement;

FIG. 8 illustrates the ion source of FIGS. 3-6 installed in a retrofitvolume of a pre-existing ion implanter while FIG. 8A illustrates, on asmaller scale, the entire implanter of FIG. 8;

FIG. 9 is a diagram of the operator interface of a conventional Bernasarc discharge ion source while FIG. 9A is a similar view of a Bernassource with indirectly heated cathode;

FIG. 10 is a diagrammatic illustration of a semiconductor device,illustrating standard CMOS ion implantation applications.

FIG. 11 is a top view of an aperture plate that has provisions forreceiving a bias voltage relative to the voltage of the remaining wallsof an ionization chamber, while FIGS. 11A and 11B, taken on respectivelines in FIG. 11, are side views respectively of the inside face of theaperture plate, facing the interior of the ionization chamber and theoutside face, directed toward the extraction optics.

FIG. 11C is an edge view of an aperture plate illustrating it's mountingto the main body of the ionization chamber by insulating stand offs.

FIGS. 12A and 12B are side views of the inside face and outside face ofan aperture insert plate of another embodiment.

FIG. 12C is a side view of an insulator frame into which the insertplate of FIGS. 20A and 20B may be mounted.

FIG. 13 is a cut-away view of an ion source configured with an externalcathode in accordance with the present invention showing the electronflow from the external cathode through the ionization chamber to a beamdump or repeller.

FIG. 14 is a graphical illustration of the extraction current as afunction of the external cathode current for the embodiment illustratedin FIG. 13.

FIG. 15 is an alternate embodiment of the ion source illustrated in FIG.13.

FIG. 16 is a graphical illustration of the extraction current as afunction of the gap between the external cathode and the ionizationchamber for different cathode currents for the embodiment illustrated inFIG. 13.

FIG. 17 is another alternate embodiment of the ion source illustrated inFIG. 13, illustrating the external cathode juxtaposed at a firstposition d₁ for operation of the ion source in a first mode of operationand also illustrating the cathode juxtaposed at a second position d₂ foroperation of the ion source in a second mode of operation.

DETAILED DESCRIPTION

The present invention relates to an ion source which includes a singleelectron emitter that is configured with a cathode mounted external tothe ionization chamber for use in fabrication of semiconductors. Theionization chamber is formed with an electron entrance aperture in onewall thereof. In accordance with one aspect of the invention, thecathode portion of the electron emitter, which may be either directlyheated or indirectly heated, is located external to the ionizationchamber. The cathode is juxtaposed so that emitted electrons arereceived by the electron entrance aperture and directed into theionization chamber in order to ionize feed gases or vapors within theionization chamber. In accordance with an important aspect of thepresent invention, the anode or other electron optics normallyassociated with the electron emitter are eliminated to enable thedistance, i.e. gap, between the electron emitter and the ionization tobe shortened. By shortening this distance, the ion source can beconfigured to operate in an arc discharge mode with an electron emitterthat is completely external to the ionization chamber, thereby removingthe entire electron emitter from the harsh plasma environment within theionization chamber.

In accordance with another important aspect of the invention, the ionsource may be operated in both a direct electron mode of operation andan arc discharge mode of operation utilizing a single electron emitter.In this embodiment, the electron emitter is selectively located at afirst distance from the ionization chamber for operation in an arcdischarge mode and at a second distance for operation in a directelectron impact mode. In this embodiment, the electron emitter may bemovably mounted and selectively secured to a desired position relativeto the ionization chamber. Alternatively, the electron emitter can berigidly mounted to a desired position. Accordingly, depending on thedistance between the cathode and the ionization chamber, the ion sourcecan be operated in a dual mode by selectively varying the distancebetween the cathode and the ionization chamber.

The salient aspects of the present invention that relate to an ionsource with a single electron emitter with an cathode external to theionization chamber operable in single mode of operation or a dual modeof operation are illustrated in FIGS. 13-17 and described below. FIGS.1-12 illustrate the general principles of an ion source that areapplicable to the present invention.

GENERAL DESCRIPTION

The ion source may include i) a vaporizer, ii) a vaporizer valve, iii) agas feed, iv) an ionization chamber, v) an electron gun, vi) a cooledmounting frame, and vii) an ion exit aperture. Included are means forintroducing gaseous feed material into the ionization chamber, means forvaporizing solid feed materials and introducing their vapors into theionization chamber, means for ionizing the introduced gaseous feedmaterials within the ionization chamber, and means for extracting theions thus produced from an ion exit aperture adjacent to the ionizationregion. In addition, means for accelerating and focusing the exitingions are provided. The vaporizer, vaporizer valve, gas feed, ionizationchamber, electron gun, cooled mounting frame, and ion exit aperture areall integrated into a single assembly in the ion source. Each of thefeatures of the ion source is described below.

Vaporizer: The vaporizer is suitable for vaporizing solid materials,such as decaborane (B.sub.10H.sub.14) and TMI (trimethyl indium), whichhave relatively high vapor pressures at room temperature, and thusvaporize at temperatures below 100 C. The temperature range between roomtemperature and 100 C is easily accommodated by embodiments in which thevaporizer is directly associated with a water heat transfer medium,while other preferred arrangements accommodate novel material whichproduce significant vapor pressures in the range up to 200 C. Forexample, solid decaborane has a vapor pressure of about 1 Torr at 20 C.Most other implant species currently of interest in the ion implantationof semiconductors, such as As, P, Sb, B, C, Ar, N, Si, and Ge areavailable in gaseous forms. However, B.sub.10 and In are not, but can bepresented in vapors from solid decaborane and TMI. The vaporizer of anembodiment of the invention is a machined aluminum block in whichresides a sealed crucible containing the solid material to be vaporized,entirely surrounded by a closed-circuit water bath, which is itselfenclosed by the aluminum block. The bath is held at a well-definedtemperature by a closed-loop temperature control system linked to thevaporizer. The closed-loop temperature control system incorporates a PID(Proportional Integral Differential) controller. The PID controlleraccepts a user-programmable temperature setpoint, and activates aresistive heater (which is mounted to a heater plate in contact with thewater bath) to reach and maintain it's setpoint temperature through athermocouple readback circuit which compares the setpoint and readbackvalues to determine the proper value of current to pass through theresistive heater. To ensure good temperature stability, a water-cooledheat exchanger coil is immersed in the water bath to continually removeheat from the bath, which reduces the settling time of the temperaturecontrol system. The temperature difference between the physicallyseparate heater plate and heat exchanger coil provides flow mixing ofthe water within the bath through the generation of convective currents.As an added mixing aid, a rotating magnetic mixer paddle can beincorporated into the water bath. Such a temperature control system isstable from 20 C to 100 C. The flow of gas from the vaporizer to theionization chamber is determined by the vaporizer temperature, such thatat higher temperatures, higher flow rates are achieved.

The flow of gas from a vaporizer to the ionization chamber is determinedby the vaporizer temperature, such that at higher temperatures, higherflow rates are achieved.

The vaporizer communicates with the ionization chamber via a relativelyhigh-conductance path between the crucible and the ionization chamber.This may be achieved by incorporating a relatively short,large-diameter, line-of-sight conduit between the two components.High-conductance gate valves (large diameter gates with a thindimensioned housing) are used in the flow path between the vaporizer andsource body, so as not to limit this conductance. By providing a highconductance for the transport of vapor to the ionization chamber, thepressure within the vaporizer and the temperature excursion required arelower than in prior vaporizers.

In one embodiment of the ion source, a relatively low conductance supplypath is achieved employing a 5 mm diameter, 20 cm long conduit,providing a conductance of about 7.times.10.sup.−2 Us between crucibleand ionization chamber. This would require a pressure within thevaporizer of about 2 Torr to establish an ionization chamber pressure ofabout 4.5 mTorr. Another embodiment employs an 8mm diameter conduit ofthe same length, providing a conductance of about 3.times.10.sup.−1 L/s,allowing a pressure within the vaporizer of 0.5 Torr to achieve the sameflow rate of material, and hence the same pressure of 4.5 mTorr withinthe ionization chamber.

The static vapor pressure of a material at a given temperature and thedynamic pressure in the vaporizer crucible during the evolution andtransport of vapor out of the crucible during operation are not thesame. In general, the steady-state dynamic pressure is lower than thestatic vapor pressure, the extent depending on the distribution ofsource material within the vaporizer crucible, in addition to otherdetails of construction. According to the invention, the conductancesare made large to accommodate this effect. In addition, in certainpreferred embodiments, the added openness of the ionization chamber tothe vacuum environment of the source housing due to electron entranceand exit ports into the ionization chamber requires about twice the flowof gaseous material as a conventional Bernas-style source. Generallyaccording to the invention, it is preferred that the conductance be inthe range of about 3.times.10.sup.−2 to 3.times.10.sup.−1 Us, preferablythe length of the conduit being no less than 30 cm while its diameter isno less than about 5 mm, the preferred diameter range being between 5and 10 mm. Within these limits it is possible to operate at much lowertemperatures than conventional vaporizers, no large addition oftemperature being required to elevate the pressure to drive the flow tothe ionization chamber. Thus the temperature-sensitive materials areprotected and a broad range of materials are enabled to be vaporizedwithin a relatively small temperature range.

In several of the embodiments of the vaporizer presented, theconstruction of the vaporizer, following these guidelines, allowsoperation at temperatures between 20 C and 100 C or 200 C. Given thehigh conductance of the vaporizer, and such temperature ranges, I haverealized that the wide range of solid source materials that can beaccommodated include some materials which have not previously been usedin ion implantation due to their relatively low melting point. (Itgenerally being preferred to produce vapors from material in solidform).

An additional advantage of enabling use of only a relatively lowpressure of vaporized gas within the crucible is that less material canbe required to establish the desired mass flow of gas than in priordesigns.

In another embodiment a different vaporizer PID temperature controlleris employed. In order to establish a repeatable and stable flow, thevaporizer PID temperature controller receives the output of anionization-type pressure gauge which is typically located in the sourcehousing of commercial ion implanters to monitor the sub-atmosphericpressure in the source housing. Since the pressure gauge output isproportional to the gas flow into the ion source, its output can beemployed as the controlling input to the PID temperature controller. ThePID temperature controller can subsequently raise or diminish thevaporizer temperature, to increase or decrease gas flow into the source,until the desired gauge pressure is attained. Thus, two useful operatingmodes of a PID controller are defined: temperature-based, andpressure-based.

In another embodiment, these two approaches are combined such thatshort-term stability of the flow rate is accomplished by temperatureprogramming alone, while long-term stability of the flow rate isaccomplished by adjusting the vaporizer temperature to meet a pressureset-point. The advantage of such a combined approach is that, as thesolid material in the vaporizer crucible is consumed, the vaporizertemperature can be increased to compensate for the smaller flow ratesrealized by the reduced surface area of the material presented to thevaporizer.

In another embodiment of the vaporizer, a fluid heat transfer medium isnot used. Rather than a water bath, the crucible is integral with themachined body of the vaporizer, and heating and cooling elements areembedded into the aluminum wall of the vaporizer. The heating element isa resistive or ohmic heater, and the cooling element is a thermoelectric(TE) cooler. The vaporizer is also encased in thermal insulation toprevent heat loss to the ambient, since the desired vaporizertemperature is typically above room temperature. In this embodiment, theheating/cooling elements directly determine the temperature of the wallsof the vaporizer, and hence the temperature of the material within thecrucible, since the material is in direct contact with the walls of thevaporizer which is e.g. machined of a single piece of aluminum. The samePID temperature controller techniques can be used as in the previouslydescribed embodiment, enabling the vaporizer to reach a temperature inexcess of 100 C, preferably up to about 200 C.

In another embodiment, the vaporizer consists of two mating, butseparate components: a vaporizer housing and a crucible. The crucible isinserted into the housing with a close mechanical fit. The surface ofthe vaporizer housing which makes contact with the crucible contains apattern of rectangular grooves, into which sub-atmospheric pressurizedconductive gas is introduced. The pressurized gas provides sufficientthermal conductivity between the crucible and the temperature-controlledhousing to control the temperature of the crucible surface in contactwith decaborane or other solid feed material to be vaporized. Thisembodiment allows the crucible to be easily replaced during service ofthe vaporizer. The same PID temperature controller techniques can beused as in the previously described embodiment.

In some embodiments, the vaporizer, while still close to the ionizationchamber, communicating with it through a high conductance path, isphysically located outside of, and removably mounted to, the mainmounting flange of the ion source and the vaporizer communicates throughthe main mounting flange to the ionization chamber located within thevacuum system.

In some embodiments, two vaporizers, independently detachable from theremainder of the ion source, are provided, enabling one vaporizer to bein use while the other, detached, is being recharged or serviced.

Vaporizer Valve

In the above described vaporizer embodiments, the vapors leave thevaporizer and enter the adjacent ionization chamber of the ion sourcethrough an aperture, which is preferably coupled to a thin, highconductance gate valve with a metal seal or other thermally conductiveseal placed between the vaporizer and ionization chamber. The gate valveserves to separate the vaporizer from the ionization chamber, so that novapor escapes from the vaporizer when the valve is shut, but a short,high-conductance line-of-sight path is established between theionization chamber and vaporizer when the valve is open, thus allowingthe vapors to freely enter the ionization chamber. With the valve in theclosed position, the vaporizer with the valve attached may be removedfrom the ion source without releasing the toxic vaporizer materialcontained in the crucible. The ion source may then be sealed byinstalling a blank flange in the position previously occupied by thevaporizer valve. In another embodiment, two isolation valves areprovided in series, one associated with the removable vaporizer and oneassociated with all of the other components of the ion source, with thedisconnect interface being located between the two valves. Thus bothparts of the system can be isolated from the atmosphere while the partsare detached from one another. One of the mating valves (preferably, thevalve isolating the ion source body) has a small, valved roughing portintegrated internal to the valve body, which enables the air trapped inthe dead volume between them to be evacuated by a roughing pump afterthe two valves are mated in a closed position. If the source housing ofthe implanter is under vacuum, the vaporizer can be installed with itsvalve in a closed state after being refilled. It is mated to the closedvalve mounted to the ion source in the implanter. The vaporizer valvecan then be opened and the vaporizer volume pumped out through theroughing port (along with the gas trapped in the dead volume between thevalves). Then the ion source valve can be opened, without requiringventing of the source housing. This capability greatly reduces theimplanter down time required for servicing of the vaporizer. In anothersystem, two such vaporizers, each with two isolation valves in series,as described, are provided in parallel, suitable to vaporize differentstarting materials, or to be used alternatively, so that one may beserviced and recharged while the other is functioning.

Gas Feed

In order to operate with gaseous feed materials, ion implanterstypically use gas bottles which are coupled to a gas distributionsystem. The gases are fed to the ion source via metal gas feed lineswhich directly couple to the ion source through a sealed VCR or VCOfitting. In order to utilize these gases, embodiments of the ion sourceof the present invention likewise have a gas fitting which couples tothe interior of the ionization chamber and connects to a gasdistribution system.

Ionization Chamber

The ionization chamber defines the region to which the neutral gas orvapor fed to the source is ionized by electron impact. In certainembodiments, the ionization chamber is in intimate thermal andmechanical contact with the high conductance vaporizer valve or valvesthrough thermally conductive gaskets, which are likewise in intimatethermal contact with the vaporizer through thermally conductive gaskets.This provides temperature control of the ionization chamber throughthermal contact with the vaporizer, to avoid heat generated in theionization chamber from elevating the temperature of the walls of thechamber to temperatures which can cause decaborane or otherlow-temperature vaporized materials or gases to break down anddissociate.

In other embodiments, the ionization chamber, as a removable component,(advantageously, in certain instances, a regularly replaced consumablecomponent) is maintained in good heat transfer relationship with atemperature-controlled body, such as a temperature controlled solidmetal heat sink having a conventional water cooling medium or beingcooled by one or more thermoelectric coolers.

The ionization chamber in some embodiments is suitable for retrofitinstallation is sized and constructed to provide an ionization volume,extraction features, and ion optical properties compatible with theproperties for which the target implanter to be retrofitted wasdesigned.

In some embodiments, the ionization chamber is rectangular, made of asingle piece of machined aluminum, molybdenum, graphite, silicon carbideor other suitable thermally conductive material. Because contact of theionization chamber with a fluid transfer medium is avoided in designspresented here, in certain instances the ionization chamber andextraction aperture are uniquely formed of low cost graphite, which iseasily machined, or of silicon carbide, neither of which creates risk oftransition metals contamination of the implant. Likewise for the lowtemperature operations (below its melting point) an aluminumconstruction may advantageously be employed. A disposable andreplaceable ionization chamber of machined graphite or of siliconcarbide is a particular feature of the invention.

The ionization chamber in some embodiments is approximately 7.5 cm tallby 5 cm wide by 5 cm deep, approximating the size and shape ofcommercially accepted Bernas arc discharge ionization chambers. Thechamber wall thickness is approximately 1 cm. Thus, the ionizationchamber has the appearance of a hollow, rectangular five-sided box. Thesixth side is occupied by the exit aperture. The aperture can beelongated as are the extraction apertures of Bernas arc discharge ionsources, and located in appropriate position in relation to the ionextraction optics. The flow rate of the gas fed into the ionizationchamber is controlled to be sufficient to maintain proper feed gaspressure within the ionization chamber. For most materials, includingdecaborane, a pressure between 0.5 mTorr and 5 mTorr in the ionizationchamber will yield good ionization efficiency for the system beingdescribed. The pressure in the source housing is dependent upon thepressure in the ionization chamber. With the ionization chamber pressureat 0.5 mTorr or 5 mTorr, the ion gauge mounted in the source housing,typically used in commercial ion implanters to monitor source pressure,will read about 1.times.10.sup.−5 Torr and 1.times.10.sup.−4 Torr,respectively. The flow rate from the vaporizer or gas feed into theionization chamber required to sustain this pressure is between about 1sccm and 10 sccm (standard cubic centimeters per minute).

Electron Gun

Except as mentioned below, the general principles of an electron gunsuitable for use with the invention illustrated in FIGS. 13-17 aregenerally described below. However, those embodiments of the electrongun, for example, as illustrated in FIGS. 4, 4B, 4C, which illustrate ananode 34 or other electron optics between the cathode 33 and theelectron entrance aperture 35 are not applicable to the inventionillustrated in FIGS. 13-17 and are provided to provide a betterunderstanding of the invention.

For ionizing the gases within the ionization chamber, electrons ofcontrolled energy and generally uniform distribution are introduced intothe ionization chamber by a broad, generally collimated beam electrongun as shown in the illustrative figures described below. A high-currentelectron gun is mounted adjacent one end of the ionization chamber,external to that chamber, such that a directed stream of primaryenergetic electrons is injected through an open port into the ionizationchamber along the long axis of the rectangular chamber, in a directionparallel to and adjacent the elongated ion extraction aperture. Thecathode of the electron gun is held at an electric potential below thepotential of the ionization chamber by a voltage equal to the desiredelectron energy for ionization of the molecules by the primaryelectrons. Two ports, respectively in opposite walls of the ionizationchamber are provided to pass the electron beam, one port for entrance ofthe beam as mentioned above, and the second port for exit of the beamfrom the ionization chamber. After the electron beam exits theionization chamber, it may be intercepted by a beam dump located justoutside of the ionization chamber; the beam dump being aligned with theelectron entry point, and preferably maintained at a potential somewhatmore positive than that of the ionization chamber. The electron beam isof an energy and current that can be controllably varied over respectiveranges to accommodate the specific ionization needs of the various feedmaterials introduced into the ionization chamber, and the specific ioncurrents required by the ion implant processes of the end-user. Inparticular embodiments, the electron gun is constructed to be capable ofproviding electron beam energy programmable between 20 eV and 500 eV.

The lowest beam energies in this energy range accommodate selectiveionization of a gas or vapor below certain ionization thresholdenergies, to limit the kinds of end-product ions produced from theneutral gas species. An example is the production ofB.sub.10H.sub.x.sup.+ ions without significant production ofB.sub.9H.sub.x.sup.+, B.sub.8H.sub.x.sup.+, or other lower-order boranesfrequently contained in the decaborane cracking pattern when higherelectron impact energies are used.

The higher beam energies in the energy range of the electron gun areprovided to accommodate the formation of multiply-charged ions, forexample, As.sup.++ from AsH.sub.3 feed gas. For the majority of ionproduction from the various feed gases used in semiconductormanufacturing, including the production of B.sub.10H.sub.x.sup.+ fromdecaborane, an electron beam energy between 50 eV and 150 eV can yieldgood results.

In some embodiments, the electron gun is so constructed that theelectron beam current can be selected over a range of injected electronbeam currents between 0.1 mA and 500 mA, in order to determine the ioncurrent extracted from the ion source in accordance with the implantdemand. Control of electron current is accomplished by a closed-loopelectron gun controller which adjusts the electron emitter temperatureand the electron gun extraction potential to maintain the desiredelectron current setpoint. The electron emitter, or cathode, emitselectrons by thermionic emission, and so operates at elevatedtemperatures. The cathode may be directly heated (by passing an electriccurrent through the cathode material), or indirectly heated. Cathodeheating by electron bombardment from a hot filament held behind thecathode is an indirect heating technique well-practiced in the art. Thecathode may be made of tungsten, tantalum, lanthanum hexaboride(LaB.sub.6), or other refractory conductive material. It is realizedthat LaB.sub.6 offers a particular advantage, in that it emits copiouscurrents of electrons at lower temperatures than tungsten or tantalum.As discussed further below, the preferred separate mounting of theelectron beam gun, thermally isolated from the ionization chamber, is anadvantageous factor in keeping the ionization chamber cool. Electronbeam guns having cathodes mounted close to the ionization chamber on acooled support, which discharge directly into the chamber, are shown inthe first two embodiments described below.

The electron beam, however produced, has a significant cross-sectionalarea, i.e. it is a broad generally collimated beam as it transits theionization chamber, to the beam dump with which it is aligned. Inpreferred embodiments, the electron beam within the ionization chamberhas a generally rectangular cross section, e.g. in one embodimentapproximately 13 mm.times.6 mm as injected into the ionization chamber,to match with a relatively wide extraction aperture of a high currentmachine, or the rectangular cross section is e.g. of a squarecross-section profile for use with a narrower ion extraction aperture.In the case of direct injection, the shape of the injected electron beamcan be determined by the shape of the electron optics, e.g. the grid andanode apertures of an electron gun, which, for example, may both beapproximately 13 mm.times.6 mm, and also by the shape of the cathode orelectron emitter, which, for the first example given, is somewhat largerthan the grid and anode apertures, approximately 15 mm.times.9 mm. Theadvantage of generating a generally rectangular electron beam profile isto match the conventionally desired ion beam profile as extracted fromthe ion source, which is also rectangular: The rectangular exit aperturefrom which the ion beam is extracted is approximately 50 mm tall by 3.5mm wide in many high-current implanters; in such cases the electron beam(and thus the ions produced by electron impact) can present a profile tothe exit aperture within the ionization chamber of approximately 64mm.times.13 mm. If the end-user wishes, an enlarged exit aperture may beemployed to obtain higher extracted currents.

As mentioned above, in the walls of the ionization chamber, there areboth an electron entrance port and an aligned electron exit port for theelectron beam, which departs from the conventionally employed Bernas ionsource. In Bernas ion sources. energetic electrons produced by anemitter located typically internal to the ionization chamber strike thewalls of the chamber to form the basis of an “arc discharge”. Thisprovides a substantial heat load which elevates the temperature of theionization chamber walls. In the present invention, the ionizingelectrons (i.e the energetic or “primary” electrons) pass through theionization chamber to the defined beam dump, substantially withoutintercepting the general chamber walls. “Secondary” electrons, i.e.low-energy electrons produced by ionization of the feed gas, still canreach the general walls of the ionization chamber but since these arelow energy electrons, they do not provide significant heat load to thewalls. The feature of through-transit of the primary electrons allowsthe ionization chamber to be conductively cooled, e.g. by the vaporizer,or by a cooled block against which the ionization chamber is mounted insubstantial thermal contact, without providing a large heat load on thetemperature controller of the vaporizer or block. To avoid the heatgenerated by the electron gun and the energetic electron beam, theelectron gun and the electron beam dump are mounted in thermallyisolated fashion, preferably either or both being mounted on respectivewater-cooled parts of a cooled mounting frame. This frame is dynamicallycooled, e.g. by high-resistivity, de-ionized water commonly available incommercial ion implanters.

Electron Repeller

To maximize the source ionization efficiency, anti cathodes may bedisposed at the opposite end of the ionization chamber from the electronemitter. The anti cathode typically operates at the same potential asthe emitter, thus reflecting the electron beam back and forcing multiplepasses of electrons through the ionization chamber. Alternatively, amagnetic reflector can be used. When charged particles travel towards avolume of increasing magnetic flux density, a condition exists wheremost of the electrons are reflected back into the direction where theycame from. This effect is called magnetic mirror effect and is wellcharacterized phenomena in plasma physics. To use this idea instead ofan electrostatic anti cathode, a permanent magnet can be place at theopposite side of the ionization chamber from the emitter. This creates astrong magnetic field towards the end of the ionization chamber thusreflecting most of the ionizing electrons back towards the emitter endof the chamber. The magnetic repeller has the advantage of being in thesame potential as it's surroundings, thus making it more reliable as anelectrostatic anti cathode.

Cooled Mounting Frame and Beam Dump

The cooled mounting frame is e.g. a water-cooled sheet metal assembly onwhich the electron gun and an electron beam dump may be mounted. Thebeam dump may be used alternatively to the electron repeller. The frameconsists of two separate mechanical parts which allow the electron gunand a beam dump to be independently biased. By mounting these twocomponents to this frame, a heat load to the ionization chamber can besubstantially avoided. The frame provides a mechanical framework for thethus-mounted components, and in addition the frame and the mountedcomponents can be held at an electric potential different from thepotential of the ionization chamber and vaporizer by mounting to the ionsource assembly on electrically insulating standoffs.

In embodiments discussed below, the beam dump is discretely defined andisolated, preferably being removed from direct contact with theionization chamber, with the electron beam passing through an exit portin the ionization chamber prior to being intercepted by the beam dump.The beam dump can readily be maintained at a potential more positivethan the walls of the chamber to retain any secondary electrons releasedupon impact of primary electrons up on the beam dump. Also, the beamdump current can be detected for use in the control system as well asfor diagnostics. Also, in a multi-mode ion source, by being electricallyisolated, the voltage on the dump structure can be selectively changedto negative to serve an electron-repeller (anticathode) function, asdescribed below.

In another construction, the distinctly defined beam dump though can bein physical contact with the exit port in such a way that thermalconduction between the cooled beam dump and the exit port is poor e.g.,by point contact of discrete elements. Electrical insulation, which hasthermal insulation properties as well, can be provided to enable avoltage differential to be maintained while preventing heating of thegeneral walls of the ionization chamber. One advantage of thisembodiment is a reduced conduction of the source gas out of theionization chamber, reducing gas usage.

The extraction of ions from the ionization chamber is facilitated by anasymmetric relationship of the electron beam axis relative to thecentral chamber axis, locating the site of ionization closer to theextraction aperture. By maintaining a voltage on the aperture platethrough which the ions are extracted that is lower than that of theother chamber walls, the ions are drawn toward the extraction path.

In use of the ion source in a mode different from that used fordecaborane as described above, e.g. using BF.sub.3 feed gas, theelectron beam dump may be biased to a negative potential relative to theionization chamber, e.g. to a voltage approximating that of the cathodepotential, in a “reflex geometry” whereby the primary electrons emittedby the electron gun are reflected back into the ionization chamber andto the cathode, and back again repeatedly, i.e. instead of serving as abeam dump, in this mode the dump structure serves as a “repeller”, oranticathode. An axial magnetic field may also be established along thedirection of the electron beam by a pair of magnet coils external to theion source, to provide confinement of the primary electron beam as it isreflected back and forth between the cathode and beam dump. This featurealso provides some confinement for the ions, which may increase theefficiency of creating certain desired ion products, for example B.sup.+from BF.sub.3 feed gas. Such a reflex mode of operation is known per seby those practiced in the art, but is achieved here in a uniquemulti-mode ion source design capable of efficiently producing e.g.decaborane ions.

A multimode ion source includes an electron gun for the purposes asdescribed, disposed coaxially within a magnet coil that is associatedwith the source housing and ionization chamber contained within.

FIG. 3 shows in schematic an embodiment of ion source 1. The vaporizer 2is attached to the vaporizer valve 3 through an annular thermallyconductive gasket 4. The vaporizer valve 3 is likewise attached to themounting flange 7, and the mounting flange 7 is attached to ionizationchamber body 5 by further annular thermally conductive gaskets 6 and 6A.This ensures good thermal conduction between the vaporizer, vaporizervalve, and ionization chamber body 5 through intimate contact viathermally conductive elements. The mounting flange 7 attached to theionization chamber 5, e.g., allows mounting of the ion source 1 to thevacuum housing of an ion implanter, (see FIG. 8) and contains electricalfeedthroughs (not shown) to power the ion source, and water-coolingfeedthroughs 8, 9 for cooling. In this preferred embodiment, waterfeedthroughs 8, 9 circulate water through the cooled mounting frame 10to cool the mounting frame 10 which in turn cools the attachedcomponents, the electron beam dump 11 and electron gun 12. The exitaperture plate 13 is mounted to the face of the ionization chamber body5 by metal screws (not shown). Thermal conduction of the ion exitaperture plate 13 to the ionization chamber body 5 is aided byconductive annular seal 14 of metal or a thermally conductive polymer.

When the vaporizer valve 3 is in the open position, vaporized gases fromthe vaporizer 2 can flow through the vaporizer valve 3 to inlet channel15 into the open volume of the ionization chamber 16. These gases areionized by interaction with the electron beam transported from theelectron gun 12 to the electron beam dump 11. The ions produced in theopen volume can then exit the ion source from the exit aperture 37,where they are collected and transported by the ion optics of the ionimplanter.

The body of vaporizer 2 is made of machined aluminum, and houses a waterbath 17 which surrounds a crucible 18 containing a solid feed materialsuch as decaborane 19. The water bath 17 is heated by a resistive heaterplate 20 and cooled by a heat exchanger coil 21 to keep the water bathat the desired temperature. The heat exchanger coil 21 is cooled byde-ionized water provided by water inlet 22 and water outlet 23. Thetemperature difference between the heating and cooling elements providesconvective mixing of the water, and a magnetic paddle stirrer 24continuously stirs the water bath 17 while the vaporizer is inoperation. A thermocouple 25 continually monitors the temperature of thecrucible 18 to provide temperature readback for a PID vaporizertemperature controller (not shown). The ionization chamber body 5 ismade of aluminum, graphite, silicon carbide, or molybdenum, and operatesnear the temperature of the vaporizer 2 through thermal conduction. Inaddition to low-temperature vaporized solids, the ion source can receivegases through gas feed 26, which feeds directly into the open volume ofthe ionization chamber 16 by an inlet channel 27. Feed gases providedthrough channel 27 for the ion implantation of semiconductors includeAsH.sub.3, PH.sub.3, SbF.sub.5, BF.sub.3, CO.sub.2, Ar, N.sub.2,SiF.sub.4, and GeF.sub.4, and with important advantages GeH.sub.4,SiH.sub.4, and B.sub.2H.sub.6, described below. When the gas feed 26 isused to input feed gases, the vaporizer valve 3 is closed. In the caseof a number of these gases, the broad beam electron ionization of thepresent invention produces a mid-to-low ion current, useful formid-to-low dose implantations. For higher doses, an embodiment capableof switching mode to a reflex geometry, with magnetic field, can beemployed.

The vaporizer 2 of FIG. 3, or that of FIG. 3A to be described, can bedemounted from the ion source 1 by closing the vaporizer valve 3 andremoving the unit at seal 6, (parting line D), compare FIGS. 3B and 3C.This is useful for recharging the solid feed material in the crucible18, and for maintenance activities.

In the embodiment of FIG. 3D, two valves, 3 and 3A are provided inseries, valve 3 being permanently associated, as before, with removablevaporizer 28 and valve 3A being permanently associated with mountingflange 7, with the demounting plane D disposed between the two valves.

In the embodiment of the ion source shown in FIG. 3A, the vaporizer 28is of a different design from that of FIG. 3, while the rest of the ionsource is the same as in FIG. 3. In vaporizer 28, there is no water bathor water-fed heat exchanger. Instead, the volume occupied by water bath17 in FIG. 3 is occupied by the machined aluminum body 29 of vaporizer28. A resistive heater plate 20 is in direct thermal contact with thevaporizer body 29 to conductively heat the body 29, and a thermoelectric(TE) cooler 30 is in direct thermal contact with the vaporizer body 29to provide conductive cooling. A thermally insulating sleeve 31surrounds the vaporizer 28 to thermally insulate the vaporizer fromambient temperature. If desired, several heater plates 20 and TE coolers30 can be distributed within the vaporizer body 29 to provide moreconductive heating and cooling power, and also to provide a morespatially uniform temperature to the crucible. This construction permitsthe vaporizer to operate at temperatures in excess of 100 C, up to about200 C.

FIG. 3B illustrates an embodiment in which successive mounting flangesof the series of vaporizer 28, isolation valve 3 and the ion source 1,are of increasing size, enabling access to each flange for detachment.Mounting flange 70 enables bolt-on of the assembled ion source to theion source housing, see e.g. FIG. 8. Mounting Flange 7 a enablesattachment and detachment of the vaporizer 28 and its associated valve 3from flange 7 at parting line D, see FIG. 3C. Mounting Flange 7 benables detachment of the valve 3 from the main body of the vaporizerfor maintenance or recharging the vaporizer.

The embodiment of FIG. 3D has two valves 3 and 3 a, valve 3 normallystaying attached to the vaporizer and valve 3 a normally attached to ionsource mounting flange 7. These enable isolation of both the vaporizer28 and the ion source 1 before demounting the vaporizer at parting lineD. The body of mated valve 3 a includes roughing passage 90 connected byvalve 92 to roughing conduit 91 by which the space between the valvesmay be evacuated, and, upon opening valve 3, by which the vaporizer maybe evacuated prior to opening valve 3 a. Thus attachment of vaporizer 28need not adversely affect the vacuum being maintained in the ion sourceand beam line.

The vent line 93, and associated valve 94 enables relief of vacuumwithin the vaporizer prior to performing maintenance and as well may beused to evacuate and outgas the vaporizer after recharging, to conditionit for use.

The embodiment of FIG. 3E illustrates a dual vaporizer construction,having the capabilities previously described. The vapor passage 15 inmetal block heat sink 5 a bifurcates near mounting flange 7, thebranches 15′ leading to respective demountable vaporizers VAP1 and VAP2,each having two isolation valves separable at parting line D. Theionization chamber body 5 b is of discrete construction, demountablymounted in intimate heat transfer relationship to temperature controlledmounting block 5 a.

Separate coolant passage 66 and 67 telescopically receive so-calledsquirt tubes which centrally conduct cold, deionized water to the deadend of the passage. The emerging cooled water has its maximum effectthat that point, in the outward regions of respectively the mountingblock 5 a and the cooled frame 10, the water returns through the annularspace defined between the exterior of the squirt tube and the passage inwhich the tube resides.

FIG. 3F shows a vaporizer similar to that of FIG. 3A, but instead of aone-piece aluminum construction, the body of the vaporizer has twomating, but separate components: a vaporizer housing 29.sup.1 and acrucible 18.sup.1. The crucible is inserted into the housing 29.sup.1with a close mechanical fit. The surface of the vaporizer housing whichmakes contact with the crucible contains a pattern of rectangulargrooves, into which pressurized gas (typically at sub-atmosphericpressure) is introduced through gas inlet 93.sup.1. The pressurized gasprovides sufficient thermal conductivity between the crucible 18.sup.1and the temperature-controlled housing 291 to control the temperature ofthe crucible surface 65 in contact with decaborane or other solid feedmaterial 19 to be vaporized. This embodiment allows the crucible 181 tobe easily replaced during service of the vaporizer. Gas is also fed intothe volume surrounding heat exchanger 21, to promote thermal conductionbetween the heat exchanger 21 and the housing 29.sup.1. The heatexchanger 21 is shown as a water-fed coil, but may alternativelycomprise a TE cooler, such as cooler 30 in FIG. 3A.

The electron beam passes through a rectangular entrance port 35 (FIG. 4)in the ionization chamber and interacts with the neutral gas within theopen volume 16, defined within the ionization chamber body 5. The beamthen passes directly through a rectangular exit port 36 in theionization chamber and is intercepted by the beam dump 11, which ismounted on the water-cooled mounting frame 10. Beam dump 11 ismaintained at a positive potential relative to the electron gun, andpreferably slightly positive relative to the walls of the ionizationchamber as well. Since the heat load generated by the hot cathode 33 andthe heat load generated by impact of the electron beam 32 with the beamdump 11 is substantial, their location outside of the ionization chamberopen volume 16 prevents their causing dissociation of the neutral gasmolecules and ions. The only heat load from these elements to theionization chamber is limited to modest radiation, so the ionizationchamber can be effectively cooled by thermal conduction to the vaporizer2 (FIG. 3) or by conduction to a massive mounting block 5 a (FIG. 3E).Thus, the general walls of the ionization chamber can be reliablymaintained at a temperature below the dissociation temperature of theneutral gas molecules and ions. For decaborane, this dissociationtemperature is about 350 C. Since the ion exit aperture 37 in plate 13,shown in FIGS. 4B, 5 and 6, is a generally rectangular aperture, thedistribution of ions created adjacent to the aperture by the broad,collimated beam of generally uniformly dispersed electrons should belikewise uniform. In the ionization of decaborane and other largemolecules, according to this embodiment, an arc plasma is not sustained,but rather the gas is ionized by direct electron-impact ionization bythe primary (energetic) electrons, in the absence of containment by anymajor confining magnetic field. The absence of such magnetic fieldlimits the charge-exchange interactions between the ions and relativelycool secondary electrons as they are not strongly confined as they arein an arc plasma (confined secondary electrons can cause loss of theions of interest through multiple ionizations). The decaborane ions aregenerated in the widely distributed electron beam path. This reduces thelocal ion density relative to other conventional ion sources known inthe art.

The absence of magnetic field can improve the emittance of the extractedion beam, particularly at low (e.g., 5 keV) extraction energy. Theabsence of an arc plasma as in a Bernas source also can improveemittance since there is no plasma potential present in the ionizationand extraction region. (I recognize that the presence of an arc plasmapotential in conventional plasma-based ion sources introduces asignificant random energy component to the ions prior to beingextracted, which translates directly into an added angular spread in theextracted ion beam. The maximum angular spread .theta. due to a plasmapotential .phi. is given by: .theta.=2 arcsin {.phi./E}.sup.1/2, where Eis the beam energy. For example, for a plasma potential of 5 eV and abeam energy of 5 keV, .theta.=2.5 deg. In contrast, the random energy ofions produced by direct electron-impact ionization is generally thermal,much less than 1 eV.)

FIG. 4A shows a top view of the electron exit port 36 in the open volume16 of ionization chamber body 5, and its proximity to the ion exitaperture 37 in aperture plate 13. To enable the ions to be removed fromthe ionization chamber by penetration of an electrostatic extractionfield outside of the ion source 1 through the ion exit aperture 37, theelectron beam 32 and electron exit port 36 are situated close to theexit aperture plate 13 and its aperture 37. For example, a separation ofbetween 6 mm and 9 mm between the edge of the ionization region and theion extraction aperture can result in good ion extraction efficiency,the efficiency improving with larger width extraction apertures.Depending upon the particular parameters chosen, the broad, collimatedelectron beam 32 may not fully retain its rectangular profile due toscattering, and also due to space charge forces within the electron beam32. The electron exit port 36 is sized appropriately in accordance withsuch design choices to allow passage of the electron beam withoutsignificant interception by the general walls of the ionization chamberbody 5. Thus, in certain advantageous instances, port 36 is larger thanport 35 so that it is aligned to receive and pass at least most of theresidual electron beam.

The embodiment of FIG. 4B illustrates a discretely defined beam dump 11′which is sized and shaped to fit within port 36′ such that its inner,electron receiving surface lies flush with the inner surface of thesurrounding end wall of the chamber body 5. Beam dump 11′ is mountedupon and is cooled by cooled frame 10, as before. As shows, a clearancespace c, e.g., of 1 mm, is maintained betaeen the beam dump structureand the wall of the chamber. Preferably, as shown, the structures arecooperatively shaped as in a labyrinth L.sub.s to limit the outflow ofthe dopant gas or vapor, while maintaining thermal and electricalisolation of the dump structure 11′ from the walls of the ionizationchamber, maintaining electrical isolation of the beam dump 11′ whilepreventing loss of dopant gas or vapor.

In the embodiment of FIG. 4C electrical insulation Z fills the spacebetween the beam dump and the wall of the ionization chamber,maintaining electrical isolation of the beam dump 11′ while preventingloss of dopant gas or vapor.

Referring to FIGS. 4D and 4E, a thermoelectrically or water-cooled outerhousing H.sub.c defines a space into which a chamber-defining member 5 cof heat-conductive and electrically-conductive material is removablyinserted with close operational fit. Gas inlets G.sub.i introduceconductive gas of a subatmospheric pressure (e.g., between 0.5 and 5Torr), that is significantly higher than that of the operational vacuumV.sub.o within the overall ion source housing 49 which contains theionization chamber assembly. The conductive gas (for example, N.sub.2,Ar, or He) is introduced to the interface I.sub.f between matchingsurfaces of the housing and the chamber in regions remote from exposureof the interface to operational vacuum V.sub.o, and isolated from thevaporizer and process gas feed lines. In a preferred embodiment, thecooling gas is fed through an aluminum block or cooled housing and exitsbetween the demountable ionization chamber and the block or housing, atthe interface between them, into cooling channels machined into thealuminum block. The cooling channels have the form of linear grooves(e.g., 1 mm wide by 0.1 mm deep) which populate a significant percentageof the surface area between the two mating components. This constructionallows the flat mating surfaces (the grooved aluminum surface and theflat surface of the separate ionization chamber) of the two componentsto mate flush with one another. Simple elastomeric o-rings encompass thesurface area which contains the cooling channel grooves, ensuring thatthe gas confined to the cooling channels is isolated from regions whichcontain feedthroughs and passages for process gas or vapor within thisinterface, and also isolates the cooling gas from the ionization volumeand from the vacuum housing. The spacing between those surfaces and thepressure of the conductive gas in the interface are so related that themean-free path of the conductive gas molecules is of the order of orless than the spacing of opposed surface portions at the interface.

The conductive gas molecules, by thermal motion, conduct heat across theinterface from the chamber wall to the surrounding cooled housingelements. Any regions of actual physical contact between the solidmaterial of the chamber body and of an outer housing element likewisepromotes cooling by conduction. It is to be noted that the mode ofconductive gas cooling described here does not depend upon convectionalgas flow, but only upon the presence in the interface of the gasmolecules. Therefore, in some embodiments, it may be preferred to formseals at the interface to capture the gas, as discussed above, althoughin other embodiments exposure of the interface at edges of the assemblywith leakage to the operational vacuum V.sub.o can be tolerated just asis the case with respect to cooling of semiconductor wafers asdescribed, e.g., in the King U.S. Pat. No. 4,261,762.

In other embodiments, the cooling housing of the ionization chamberassembly or similar side wall elements of other structures of the ionsource are water-cooled in the manner of cooling the mounting frame 10as described herein. In some embodiments, depending upon the heat loadon the ionization chamber, the heat conduction resulting from theinclusion of thermally conductive gasket seals, as well as regions ofphysical point contact between the matching surfaces of the chamber andhousing elements is sufficient to keep the chamber within the desiredtemperature range, and the conductive gas-cooling feature described isnot employed.

It is recognized that the heat-transfer relationships described herehave general applicability throughout the ion source and the otherstructural components of the implanter as well. Thus, the temperature ofthe vaporizer may be controlled by the heat transfer from a disposablecrucible to surrounding elements via gas conduction at an interface, foroperating conditions which require less than, for example, 2W/cm.sup.2of heat transfer through the gas interface. Likewise, surfaces of theelectron gun, the electron beam dump, the mounting frame and theaperture plate may serve as conductors via a conductive gas interface totemperature-control elements such as the thermoelectrically orwater-cooled housing that has been described, as illustrated in FIG. 4E.

FIGS. 4F and 5 show different sizes of a broad, collimated electron beampassing through the ionization chamber, the profiles of these beamsmatched in profile to the wide and narrower apertures of the respectiveionization chambers of FIGS. 4F and 5.

FIG. 6 shows the ion exit aperture plate 13 with the axis of the ionbeam directed normal to the plane of the paper. The dimensions of theexit aperture plate conform to the dimensions of the ionization chamberwithin body 5, approximately 7.6 cm tall.times.5.1 cm wide. The exitaperture plate contains an opening 37 which is approximately 5.1 cm inheight, s, by 1.3 cm wide, r, suitable for high current implanters, andhas a bevel 38 to reduce strong electric fields at its edges. It ismatched by a broad, collimated electron beam having width g of 19 mm anddepth p of 6 mm, cross-sectional area of 114 square mm. The aperture ofthe embodiment of FIG. 5, has similar features but a much narrowerwidth, e.g. a width r.sup.1, 4 mm, matched by an electron beam of widthg.sup.1 6 mm and a depth p.sup.1 of 6 mm.

FIG. 7 shows the shape of the cathode 33, or electron emitter. In apreferred embodiment, it defines a planar emitting surface, it'sdimensions being roughly 15 mm long.times.9 mm.times.3 mm thick. It canbe directly heated by passing an electric current through it, or it canbe indirectly heated, as shown, with an electric current flowing throughfilament 39 via leads 40, heating it to emit thermionic electrons 41. Bybiasing the filament 39 to a voltage several hundred volts below thepotential of cathode 33, thermionic electrons 41 heat the cathode 33 byenergetic electron bombardment, as is known in the art.

FIG. 8 illustrates the assembly of an ion source according to FIG. 3Ainto a retrofit volume 60 of a previously installed ion implanter whileFIG. 8A illustrates the complete ion implanter.

In this particular embodiment nothing has been disturbed except that theBernas ion source for which the implanter was originally designed hasbeen removed and, into the vacated volume 60, the ion source of FIG. 3Ahas been installed, with its flange 7 bolted to the ion source housingflange. The extraction electrodes 53 remain in their original position,and the new ion source presents its aperture 37 in the same region asdid the arc discharge Bernas source. The magnet coils 54 are shownremaining, available e.g., for operation in reflex mode if desired, orfor applying a containment field for electrons proceeding to the beamdump 11.

Water Cooled Block and Demountable Ionization Chamber

In the embodiment of FIG. 3E, the ionization volume 16′ is defined by ademountable end module 5 b which is mounted with conductive thermalcontact on the end of solid mounting block 5 a via thermally conductiveseal 6″.

For achieving demountability, the conductive seal 6″ is compressed viametal screws through mating surfaces of the block 5 a and thedemountable end module 5 b. This construction enables the member 5 bdefining the ionization chamber 16′ to be removed from the block 5 a andreplaced with an unused member, advantageously of disposableconstruction. It also enables a different, and in some cases moreefficient cooling of walls of the ionization chamber 16′ than inprevious embodiments. For construction of the demountable member, inaddition to aluminum (which is inexpensive and less injurious to thewafers being implanted than molybdenum, tungsten or other metals iftransported to the wafer in the ion beam), the ionization chamber memberSb and exit aperture plate 13 are advantageously constructed fromgraphite or SiC, which removes altogether the possibility of metalscontamination of the wafer due to propagation from the ion source. Inaddition, demountable ionization chambers of graphite and SiC may beformed cheaply, and thus can be discarded during maintenance, being lessexpensive to be replaced than a one-piece structure.

In another embodiment, for conductively controlling the temperature ofthe block 5 a and the chamber body 5 b, they have mating smoothsurfaces, the surface of the block containing machined cooling channelswhich admit conductive cooling gas between the block 5 a and the chamberbody 5 b, so that that gas, introduced under vacuum, transfers heat byheat conduction (not convection) in accordance with the abovedescription of FIGS. 4D and 4E, and cooling techniques used for thedifferent situation of cooling wafers that are being implanted, see KingU.S. Pat. No. 4,261,762. In this case, gaskets at the vapor and gaspassages prevent mixing of the conductive heat transfer gas, such asargon, with the gas or vapor to be ionized.

As shown, block 5 a is cooled by water passages 24 a, either associatedwith its own thermal control system, FIG. 3E or associated with thecooling system 24 that cools frame 10 on which the beam dump 11 ismounted. By being based upon heat conduction through solid members,water contact with the walls of the ionization chamber is avoided,making it uniquely possible to fabricate the ionization chamber ofmaterials, such as low cost machined or molded graphite, which cannotconveniently be exposed to water. The remote location of the cathode andits heat effects combine with these mounting features to achieve desiredcool-running of the ionization chamber.

Electron Injection for High Current Applications

For some ion implant applications, it is desired to obtain an ioncurrent approaching the highest ion currents of which the technology iscapable. This depends critically on the value of electron beam currenttraversing the ionization chamber, since the ion current produced isroughly proportional to the value of this electron current. The electroncurrent injected into the ionization chamber is limited by the effectsof space charge forces that act on the electron trajectories within theelectron gun optics and the ionization chamber. In the space chargelimit, these forces can add an increased width to a tightly focused beamwaist produced by a lens, and can introduce an increased angulardivergence to a beam as it diverges downstream of the waist.

The maximum electron current which can be transported through a tube ofdiameter D and length L can be produced by focusing the beam on a pointat the center of the tube with an angle a=D/L expressed in radians. Insuch case, the maximum current is given by: l.sub.max=0.0385V.sup.3/2a.sup.2 (1)

where l.sub.max is the electron current measured in mA, and V is thevoltage in volts corresponding to electrons of energy E=eV, where e isthe electronic charge. Also, in this example the minimum waist diameterw is given by w=0.43 D. Inserting a=15.degree. and V=100V into equation(1) yields l.sub.max=10 mA, whereas inserting a=5.degree. and V=100Vyields l.sub.max=106 mA.

Referring to the embodiment of FIGS. 11-11B, biasing of the apertureplate is accomplished by forming it of an insulating material such asboron nitride, coating the exterior and interior surfaces which areexposed to the ions with an electrically conductive material such asgraphite, and electrically biasing the conductor.

In other embodiments insulator standoffs are employed, see FIG. 11C, tojoin the electrically conductive extraction aperture plate to thechamber while maintaining its electrical independence. In embodiments ofthis feature, gas loss from the ionization chamber at the edges of theaperture plate can be minimized by interfitting conformation of theedges of the electrically isolated aperture plate and the body of theionization chamber (involuted design) to effect labyrinth seal effectssuch as described in relation to FIG. 4B.

In accordance with the embodiment of FIGS. 12A, B and C, an electricallyconductive aperture plate insert is mounted in an electricallyinsulating frame which holds the aperture plate in place, and providesan electrical contact to the insert.

The embodiment facilitates change of aperture plates in accordance withchanges of the type of implant run. In some embodiments thermoelectriccoolers may be associated with the aperture plates to keep them fromover-heating. In other embodiments, an extension of cooled frame 10 or aseparate cooled mounting frame is employed to support the apertureplate.

Universal Ion Source Controller

A universal controller for the ion source of the invention uniquelyemploys the user interface that is used with arc discharge ion sourcessuch as the Bernas and Freeman types. FIG. 9 shows, in diagrammaticform, a typical control system 200 for operating a Bernas type ionsource. The operator for such existing machines programs the implanterthrough an Operator Interface 202 (OI), which is a set of selectablegraphical user interfaces (GUI's) that are selectively viewed on acomputer screen. Certain parameters of the implanter are controlleddirectly from the OI, by either manually inputting data or by loading apredefined implant recipe file which contains the desired parametersthat will run a specific implant recipe. The available set of GUI'sincludes controls and monitoring screens for the vacuum system, waferhandling, generation and loading of implant recipes, and ion beamcontrol.

In many implanter systems, a predetermined set of ion source parametersis programmable through the Beam Control Screen of the OI represented inFIG. 9, including user-accessible setpoint values for Arc Current, ArcVoltage, Filament Current Limit, and Vaporizer Temperature. In additionto these setpoints, the actual values of the same parameters (forexample, as indicated by the power supply readings) are read back anddisplayed to the operator on the OI by the control system.

Many other parameters that relate to the initial set up of the beam fora given implant are programmed and/or displayed through the Beam ScreenGUI, but are not considered part of the operator's ion source control.These include beam energy, beam current, desired amount of the ion,extraction electrode voltages, vacuum level in the ion source housing,etc.

As indicated in FIG. 9, a dedicated Ion Source Controller 204 reads andprocesses the input (setpoint) values from the OI, provides theappropriate programming signals to the stack of power supplies 206, andalso provides read backs from the power supplies to the OI. A typicalpower supply stack 206 shown in FIG. 9, includes power supplies for theArc, Filament, and Vaporizer Heater, power supplies 208, 210 and 212,respectively. The programming and power generation for the Source MagnetCurrent may be provided in the screen, but is typically providedseparate from the Ion Source Controller in many machines of thepresently installed fleet.

FIG. 9 a shows the same elements as FIG. 9, but for a Bernas-style ionsource of the kind which uses an indirectly-heated cathode (IHC). FIG. 9a is identical to FIG. 9, except for the addition of a Cathode powersupply 211, and its read back voltage and current. The additional powersupply is necessary because the IHC (indirectly heated cathode element)is held at a positive high voltage with respect to the filament, whichheats the IHC by electron bombardment to a temperature sufficient thatthe IHC emits an electron current equal to the Arc Current setpointvalue provided through the OI. The arc control is accomplished through aclosed-loop control circuit contained within the Ion Source Controller.

In general, the arc control of Bernas, Freeman, and IHC Bernas sourcesare accomplished through similar means, namely by on-board closed-loopcontrol circuits contained within the Ion Source Controller. In order tophysically retrofit the ion source of an existing ion implanter with anion source of the present invention, the original ion source is removedfrom the source housing of the implanter, the power cables are removed,and the Ion Source Controller 204 and the power supplies 206 or206.sup.1, i.e. the Filament Power Supply, Vaporizer Power Supply, ArcPower Supply, and Cathode Power Supply (if present) are removed from thegas box of the implanter. The Electron Beam Ion Source 1 of the presentinvention is inserted into the retrofit volume of the implanter, and theElectron Beam Ion Source Controller 220 and associated Power Supplies207 are inserted into the vacated volume of the gas box. A new set ofcables is connected. The desired mechanical configuration of the ionsource is prepared prior to installation into the source housing of theimplanter. For example, for decaborane production, a large width ionextraction aperture and a large dimension limiting aperture at the exitof the electron gun can be installed, to provide a large ionizationvolume. Additionally, if the implanter has installed a variable-widthmass resolving aperture 44, the width of that aperture may be increasedin order to pass a larger mass range of decaborane ions. Otherwise, theset-up proceeds in a conventional manner, modified according to thevarious features that are explained in the present text.

In addition to the electron beam controls that have just been explained,a temperature control mechanism is provided for the vaporizer 2. Thevaporizer is held at a well-defined temperature by a closed-looptemperature control system within the Controller 220. As has beenexplained above, the closed-loop temperature control system incorporatesPID (Proportional Integral Differential) control methodology, as isknown in the art. The PID controller accepts a temperature setpoint andactivates a resistive heater (which is mounted to a heater plate incontact with the water bath (see FIG. 3), or in heat transferrelationship with the mass of the vaporizer body 29 (FIG. 3A) to reachand maintain its setpoint temperature through a thermocouple read backcircuit. The circuit compares the setpoint and read back values todetermine the proper value of current to pass through the resistiveheater. To ensure good temperature stability, a water-cooled heatexchanger coil 21 is immersed in the water bath (in the case of thewater-cooled vaporizer of FIG. 3), or a thermoelectric (TE) cooler 30(in the embodiment of a solid metal vaporizer of FIG. 3A), or aheat-exchanger coil surrounded by heat-conducting gas (in the embodimentof a vaporizer utilizing pressurized gas to accomplish thermalconduction between the various elements as in FIG. 3F) to continuallyremove heat from the system, which reduces the settling time of thetemperature control system. Such a temperature control system is stablefrom 20 C to 200 C. In this embodiment, the flow of gas from thevaporizer to the ionization chamber is determined by the vaporizertemperature, such that at higher temperatures, higher flow rates areachieved. A similar temperature control system can be employed tocontrol the temperature of conductive block 5 a of FIG. 3E or 9B.

As has also previously been explained, in another embodiment a differentvaporizer PID temperature controller is employed. In order to establisha repeatable and stable flow, the vaporizer PID temperature controllerreceives the output of an ionization-type pressure gauge which istypically located in the source housing of commercial ion implanters tomonitor the sub-atmospheric pressure in the source housing. Since thepressure gauge output is proportional to the gas flow into the ionsource, it output can be employed as the controlling input to the PIDtemperature controller. The PID temperature controller can subsequentlyraise or diminish the vaporizer temperature, to increase or decrease gasflow into the source, until the desired gauge pressure is attained.Thus, two useful operating modes of a PID controller are defined:temperature-based, and pressure-based.

In the embodiments of FIGS. 3 and 3A, temperature of the ionizationchamber is controlled by the temperature of the vaporizer. Temperaturecontrol for the embodiment of FIG. 3E is achieved by a separatetemperature sensing and control unit to control the temperature of themetal heat sink by use of a heat transfer medium or thermoelectriccoolers or both.

Calculations of Expected Ion Current

The levels of ion current production that can be achieved with this newion source technology are of great interest. Since the ion source useselectron-impact ionization by energetic primary electrons in awell-defined sizeable ionization region defined by the volume occupiedby the broad electron beam in traversing the ionization chamber, its ionproduction efficiency can be calculated within the formalism of atomicphysics: l=l.sub.0[l−exp {−n l s}] (3)

where l.sub.0 is the incident electron current, l is the electroncurrent affected by a reaction having cross section s, n is the numberdensity of neutral gas molecules within the ionization volume, and l isthe path length. This equation can be expressed as follows: f=l−exp {−Ls pl} (4) [0269] where f is the fraction of the electron beam effectingionization of the gas, L is the number density per Torr of the gasmolecules at 0 C (=3.538.times.10.sup.16 Torr.sup.−1 cm.sup.−3), s isthe ionization cross section of the specific gas species in cm.sup.2,and pl is the pressure-path length product in Torr−cm.

The peak non-dissociative ionization cross section of decaborane has notbeen published, so far as the inventor is aware. However, it should besimilar to that of hexane (C.sub.6H.sub.14), for example, which is knownto be about 1.3.times.10.sup.−15 cm.sup.2. For an ion source extractionaperture 5 cm long and an ionization chamber pressure of2.times.10.sup.−3 Torr, equation (2) yields f=0.37. This means thatunder the assumptions of these calculations described below, 37% of theelectrons in the electron current produce decaborane ions by singleelectron collisions with decaborane molecules. The ion current(ions/sec) produced within the ionization volume can be calculated as:l.sub.ion=fl.sub.el (5) [0271] where l.sub.ion is the ion current, andl.sub.el is the electron current traversing the ionization volume. Inorder to maximize the fraction of ion current extracted from the ionsource to form the ion beam, it is important that the profile of theelectron beam approximately matches in width the profile of the ionextraction aperture, and that the ions are produced in a region close tothe aperture. In addition, the electron current density within theelectron beam should be kept low enough so that the probability ofmultiple ionizations, not taken into account by equations (3) and (4),is not significant.

The electron beam current required to generate a beam of decaborane ionscan be calculated as: l.sub.el=l.sub.ion/f (6)

Given the following assumptions: a) the decaborane ions are producedthrough single collisions with primary electrons, b) both the gasdensity and the ion density are low enough so that ion-ion andion-neutral charge-exchange interactions do not occur to a significantdegree, e.g., gas density <10.sup.14 cm.sup.−3 and ion density<10.sup.11 cm.sup.−3, respectively, and c) all the ions produced arecollected into the beam. For a 1 mA beam of decaborane ions, equation(6) yields l.sub.el=2.7 mA. Since electron beam guns can be constructedto produce electron current densities on the order of 20 mA/cm.sup.2, a2.7 mA electron beam current appears readily achievable with theelectron beam gun designs described in this application.

The density of primary electrons n.sub.e within the ionization volume isgiven by: n.sub.e=J.sub.e/e v.sub.e (7) [0275] where e is the electroniccharge (=1.6.times.10.sup.−19 C), and v.sub.e is the primary electronvelocity. Thus, for a 100 eV, 20 mA electron beam of 1 cm.sup.2cross-sectional area, corresponding to a relatively wide ion extractionaperture as illustrated in FIG. 4F, equation (7) yieldsn.sub.e.apprxeq.2.times.10.sup.10 cm.sup.−3. For a narrow extractionaperture, as illustrated in FIG. 5, a 100 eV, 20 mA of 0.4 cm.sup.2cross-sectional area would provide an electron densityn.sub.e.apprxeq.5.times.10.sup.10 cm.sup.−3. Since the ion density,n.sub.i within the ionization volume will likely be of the same order ofmagnitude as n.sub.e, it is reasonable to expect n.sub.i<10.sup.11cm.sup.−3. It is worth noting that since n.sub.e and n.sub.i areexpected to be of similar magnitude, some degree of charge neutrality isaccomplished within the ionization volume due to the ionizing electronbeam and ions being of opposite charge. This measure of chargeneutrality helps compensate the coulomb forces within the ionizationvolume, enabling higher values of n.sub.e and n.sub.i, and reducingcharge-exchange interactions between the ions.

An important further consideration in determining expected extractioncurrent levels from the broad, collimated electron beam mode is theChild-Langmuir limit, that is, the maximum space charge-limited ioncurrent density which can be utilized by the extraction optics of theion implanter. Although this limit depends somewhat on the design of theimplanter optics, it can usefully be approximated as follows:J.sub.max=1.72 (Q/A).sup.1/2U.sup.3/2d.sup.−2 (8) [0277] where J.sub.maxis in mA/cm.sup.2, Q is the ion charge state, A is the ion mass in amu,U is the extraction voltage in kV, and d is the gap width in cm. ForB.sub.10H.sub.x.sup.+ ions at 117 amu extracted at 5 kV from anextraction gap of 6 mm, equation (6) yields J.sub.max=5 mA/cm.sup.2. Ifwe further assume that the area of the ion extraction aperture is 1cm.sup.2, we deduce a Child-Langmuir limit of 5 mA ofB.sub.10H.sub.x.sup.+ ions at 5 keV, which comfortably exceeds theextraction requirements detailed in the above discussion. Ion ExtractionAperture Considerations for the Broad, Aligned Beam Electron Gun IonSource

For the broad electron beam ion source, it is possible to employ alarger width ion extraction aperture than typically employed with highcurrent Bernas arc discharge sources. Ion implanter beam lines aredesigned to image the extraction aperture onto the mass resolvingaperture, which is sized to both achieve good transmission efficiencydownstream of the mass resolving aperture, and also to maintain aspecified mass resolution R (.ident.M/.DELTA.M, see discussion above).The optics of many high-current beam lines employ unity magnification,so that, in the absence of aberrations, the extent of the ion extractionaperture as imaged onto the resolving aperture is approximatelyone-to-one, i.e., a mass resolving aperture of the same width as the ionextraction aperture will pass nearly all the beam current of a givenmass-to-charge ratio ion transported to it. At low energies, however,space charge forces and stray electromagnetic fields of a Bernas ionsource cause both an expansion of the beam as imaged onto the massresolving aperture, and also a degradation of the mass resolutionachieved, by causing significant overlap of adjacent beams of differentmass-to-charge ratio ions dispersed by the analyzer magnet.

In contrast, in the ion source the absence of a magnetic field in theextraction region, and the lower total ion current level desired, e.g.for decaborane relative say to boron, uniquely cooperate to produce amuch improved beam emittance with lower aberrations. For a given massresolving aperture dimension, this results in higher transmission of thedecaborane beam through the mass resolving aperture than one mightexpect, as well as preserving a higher R. Therefore, the incorporationof a wider ion extraction aperture may not noticeably degrade theperformance of the beam optics, or the mass resolution of the implanter.Indeed, with a wider aperture operation of the novel ion source can beenhanced, 1) because of the greater openness of the wider aperture, theextraction field of the extraction electrode will penetrate farther intothe ionization volume of the ionization chamber, improving ionextraction efficiency, and 2) it will enable use of a relatively largevolume ionization region. These cooperate to improve ion production andreduce the required density of ions within the ionization volume to makethe ion source of the invention production worthy in many instances.

Care can be taken, however, not to negatively impact the performance ofthe extraction optics of the implanter. For example, the validity ofequation (8) can suffer if the extraction aperture width w is too largerelative to the extraction gap d. By adding the preferred constraintthat w is generally equal to or less than d, then for the example givenabove in which d=6 mm, one can use a 6 mm aperture as a means toincrease total extracted ion current.

For retrofit installations, advantage can also be taken of the fact thatmany installed ion implanters feature a variable-width mass resolvingaperture, which can be employed to open wider the mass resolvingaperture to further increase the current of decaborane ions transportedto the wafer. Since it has been demonstrated that in many cases it isnot necessary to discriminate between the various hydrides of theB.sub.10H.sub.x.sup.+ ion to accomplish a well-defined shallow p-njunction (since the variation in junction depth created by the range ofhydride masses is small compared to the spread in junction locationcreated by boron diffusion during the post-implant anneal), a range ofmasses may be passed by the resolving aperture to increase ion yield.For example, passing B.sub.10H.sub.5.sup.+ throughB.sub.10H.sub.12.sup.+ (approximately 113 amu through 120 amu) in manyinstances will not have a significant process impact relative to passinga single hydride such as B.sub.10H.sub.8.sup.+, and yet enables higherdose rates. Hence, a mass resolution R of 16 can be employed toaccomplish the above example without introducing deleterious effects.Decreasing R through an adjustable resolving aperture can be arrangednot to introduce unwanted cross-contamination of the other species(e.g., As and P) which may be present in the ion source, since the massrange while running decaborane is much higher than these species. In theevent of operating an ion source whose ionization chamber has beenexposed to In (113 and 115 amu), the analyzer magnet can be adjusted topass higher mass B.sub.10H.sub.x.sup.+ or even lower massB.sub.9H.sub.x.sup.+ molecular ions, in conjunction with a properlysized resolving aperture, to ensure that In is not passed to the wafer.

Furthermore, because of the relatively high concentration of the desiredion species of interest in the broad electron beam ion source, and therelatively low concentration of other species that contribute to thetotal extracted current (reducing beam blow-up), then, though theextracted current may be low in comparison to a Bernas source, arelatively higher percentage of the extracted current can reach thewafer and be implanted as desired.

Benefits of Using Hydride Feed Gases, Etc.

Beam currents obtainable with the broad electron beam ion sourcedescribed can be maximized by using feed gas species which have largeionization cross sections. Decaborane falls into this category, as domany other hydride gases. While arc plasma-based ion sources, such asthe enhanced Bernas source, efficiently dissociate tightly-boundmolecular species such as BF.sub.3, they tend to decompose hydrides suchas decaborane, diborane, germane, and silane as well as trimethylindium, for example, and generally are not production-worthy withrespect to these materials. It is recognized, according to theinvention, however that these materials and other hydrides such asphosphene and arsine are materials well-suited to the ion sourcedescribed here (and do not present the fluorine contamination problemsencountered with conventional fluorides). The use of these materials toproduce the ion beams for the CMOS applications discussed below, usingthe ion source principles described.

For example, phosphene can be considered. Phosphene has a peakionization cross section of approximately 5.times.10.sup.−16 cm.sup.2.From the calculations above, equation (6) indicates that a broad,collimated electron beam current of 6.2 mA should yield an ion currentof 1 mA of AsH.sub.x.sup.+ ions. The other hydrides and other materialsmentioned have ionization cross sections similar to that of phosphene,hence under the above assumptions, the ion source should produce 1 mAfor all the species listed above with an electron beam current of lessthan 7 mA. On the further assumption that the transmission of theimplanter is only 50%, the maximum electron beam current required wouldbe 14 mA, which is clearly within the scope of electron beam currentavailable from current technology applied to the specific embodimentspresented above.

It follows from the preceding discussion that ion currents as high as2.6 mA can be transported through the implanter using conventional ionimplanter technology. According to the invention, for instance, thefollowing implants can be realized using the indicated feed materials inan ion source of the present invention: TABLE-US-00003 Low energy boron:vaporized decaborane (B.sub.10H.sub.14) Medium energy boron: gaseousdiborane (B.sub.2H.sub.6) Arsenic: gaseous arsine (AsH.sub.3)Phosphorus: gaseous phosphene (PH.sub.3) Indium: vaporized trimethylindium In(CH.sub.3).sub.3 Germanium: gaseous germane (GeH.sub.4)Silicon: gaseous silane (SiH.sub.4).

The following additional solid crystalline forms of In, most of whichrequire lower vaporizer temperatures than can be stably and reliablyproduced in a conventional ion source vaporizer such as is in common usein ion implantation, can also be used in the vaporizer of the presentinvention to produce indium-bearing vapor: indium fluoride (InF.sub.3),indium bromide (InBr), indium chloride (InCI and InCl.sub.3), and indiumhydroxide {In(OH).sub.3}. Also, antimony beams may be produced using thetemperature-sensitive solids Sb.sub.20.sub.5, SbBr.sub.3 and SbCl.sub.3in the vaporizer of the present invention.

In addition to the use of these materials, the present ion sourceemploying the broad, aligned electron beam in a non-reflex mode ofoperation can ionize fluorinated gases including BF.sub.3, AsF.sub.5,PF.sub.3, GeF.sub.4, and SbF.sub.5, at low but sometimes useful atomicion currents through single ionizing collisions. The ions obtainable mayhave greater ion purity (due to minimization of multiple collisions),with lessened space charge problems, than that achieved in the highercurrents produced by Bernas sources through multiple ionizations.Furthermore, in embodiments of the present invention constructed formultimode operation, all of the foregoing can be achieved in the broad,aligned electron beam mode, without reflex geometry or the presence of alarge magnetic confining field, while, by switching to a reflex geometryand employing a suitable magnetic field, a level of arc plasma can bedeveloped to enhance the operation in respect of some of the feedmaterials that are more difficult to ionize or to obtain higher, albeitless pure, ion currents.

To switch between non-reflex and reflex mode, the user can operatecontrols which switch the beam dump structure from a positive voltage(for broad, aligned electron beam mode) to a negative voltageapproaching that of the electron gun, to serve as a reseller(anticathode) while also activating the magnet coils 54. The coils,conventionally, are already present in the implanters originallydesigned for a Bernas ion source, into which the present ion source canbe retrofit. Thus a multi-mode version of the present ion source can beconverted to operate with an arc plasma discharge (in the case of ashort electron gun in which the emitter is close to the ionizationvolume as in FIGS. 4A-4D), in a manner similar to a Bernas source of thereflex type, or with a plasma without an active arc discharge if theemitter is remote from the ionization volume. In the embodimentdescribed previously the existing magnet coils can be removed andmodified magnet coils provided which are compatible with the geometry ofa retrofitted, long, direct-injection electron gun. When these magnetcoils are energized, the resultant axial magnetic field can confine theprimary electron beam (both within the electron gun and in theionization chamber) to a narrower cross-section, reducing the spreadingof the electron beam profile due to space charge, and increasing themaximum amount of useful electron current which can be injected into theionization volume. Since the electron emitter of this embodiment isremote from the ionization chamber, it will not initiate an arcdischarge, but depending on the strength of the external magnetic field,will provide a low-density plasma within the ionization region. If theplasma density is low enough, multiple ionizations induced by secondaryelectron collisions with the ions should not be significant; however,the presence of a low-density plasma may enhance the space chargeneutrality of the ionization region, enabling higher ion beam currentsto be realized.

Benefits of Using Dimer-Containing Feed Materials

The low-temperature vaporizer of the present invention canadvantageously use, in addition to the materials already mentioned,other temperature-sensitive solid source materials which cannot reliablybe used in currently available commercial ion sources due to their lowmelting point, and consequently high vapor pressure at temperaturesbelow 200 C. I have realized that solids which contain dimers of thedopant elements As, In, P, and Sb are useful in the ion source andmethods presented here. In some cases, vapors of thetemperature-sensitive dimer-containing compounds are utilized in theionization chamber to produce monomer ions. In other cases, the crackingpattern enables production of dimer ions. Even in the case ofdimer-containing oxides, in certain cases, the oxygen can besuccessfully removed while preserving the dimer structure. Use of dimerimplantation from these materials can reap significant improvements tothe dose rate of dopants implanted into the target substrates.

By extension of equation (8) which quantifies the space charge effectswhich limit ion extraction from the ion source, the following figure ofmerit which describes the easing of the limitations introduced by spacecharge in the case of molecular implantation, relative to monatomicimplantation, can be expressed:.DELTA.=n(V.sub.1/V.sub.2).sup.3/2(m.sub.1/m.sub.2).sup.−1/2 (9)

where .DELTA. is the relative improvement in dose rate achieved byimplanting a molecular compound of mass m.sub.1 and containing n atomsof the dopant of interest at an accelerating potential V.sub.1, relativeto a monatomic implant of an atom of mass m.sub.2 at an acceleratingpotential V.sub.2. In the case where V.sub.1 is adjusted to give thesame implantation depth into the substrate as the monomer implant,equation (9) reduces to .DELTA.=n.sup.2. For dimer implantation (e.g.,As.sub.2 versus As), .DELTA.=4. Thus, up to a fourfold increase in doserate can be achieved through dimer implantation. Table Ia below listsmaterials suitable for dimer implantation as applied to the presentinvention. TABLE-US-00004 TABLE IA Compound Melting Pt (deg C.) DopantPhase As.sub.2O.sub.3 315 As.sub.2 Solid P.sub.20.sub.5 340 P.sub.2Solid B.sub.2H.sub.6 B.sub.2 Gas In.sub.2(SO.sub.4).sub.3.times.H.sub.2O 250 In.sub.2 Solid Sb.sub.2O.sub.5 380 Sb.sub.2 Solid

Where monomer implantation is desired, the same dimer-containing feedmaterial can advantageously be used, by adjusting the mode of operationof the ion source, or the parameters of its operation to sufficientlybreak doan the molecules to produce useful concentrations of monomerions. Since the materials listed in Table Ia contain a high percentageof the species of interest for doping, a useful beam current of monomerdopant ions can be obtained.

Use of the Ion Source in CMOS Ion Implant Applications

In present practice, ion implantation is utilized in many of the processsteps to manufacture CMOS devices, both in leading edge and traditionalCMOS device architectures. FIG. 10 illustrates a generic CMOSarchitecture and labels traditional implant applications used infabricating features of the transistor structures (from R. Simonton andF. Sinclair, Applications in CMOS Process Technology, in Handbook of IonImplantation Technology, J. F. Ziegler, Editor, North-Holland, N.Y.,1992). The implants corresponding to these labeled structures are listedin Table I below, showing the typical dopant species, ion energy, anddose requirements which the industry expects to be in production in2001. TABLE-US-00005 TABLE I Energy Label Implant Specie (keV) Dose(cm.sup.−2) A NMOS source/drain As 30-50 1e15-5e15 B NMOS thresholdadjust (V.sub.t) P 20-80 2e12-1e13 C NMOS LDD or drain P 20-50 1e14-8e14 extension D p-well (tub) structure B 100-300 1 e13-1e14 Ep-type channel stop B 2.0-6 2e13-6e13 F PMOS source/drain B 2.0-81e15-6e15 G PMOS buried-channel V.sub.t B 10-30 2e12-1e13 H PMOSpunchthrough P 50-100 2e12-1e13 suppression l n-well (tub) structure P300-500 1e13-5e13 J n-type channel stop As 40-80 2e13-6e13 K NMOSpunchthrough B 20-50 5e12-2e13 suppression L PMOS LDD or drain B 0.5-51e14-8e14 extension M Polysilicon gate doping As, B 2.0-20 2e15-8e15

In addition to the implants listed in Table I, recent processdevelopments include use of C implants for gettering, use of Ge or Sifor damage implants to reduce channeling, and use of medium-current Inand Sb. It is clear from Table I that, apart from creating thesource/drains and extensions, and doping the polysilicon gate, all otherimplants require only low or medium-dose implants, i.e. doses between2.times.10.sup.12 and 1.times.10.sup.14 cm.sup.−2. Since the ion currentrequired to meet a specific wafer throughput scales with the desiredimplanted dose, it seems clear that these low and medium-dose implantscan be performed with the broad, aligned electron beam ion source of thepresent invention at high wafer throughput with ion beam currents below1 mA of P, As, and B. Further, of course, the decaborane ion currentsachievable according to the present invention should enable producingthe p-type source/drains and extensions, as well as p-type doping of thepolysilicon gates.

It is therefore believed that the broad, aligned electron beam ionsource described above enables high wafer throughputs in the vastmajority of traditional ion implantation applications by providing abeam current of 1 mA of B.sub.10H.sub.14, As, P, and B or B.sub.2. Theaddition of Ge, Si, Sb, and In beams in this current range, alsoachievable with the present invention, will enable more recent implantapplications not listed in Table I.

Description of External Cathode Ion Source

Features of a further embodiment of the invention are shown in FIG. 13.In this embodiment, reference is made to the electron gun in which theemitter is close to the ionization volume as in FIGS. 4A-4D. However; afeature of this embodiment in contrast to FIGS. 3 and 4A-4D. Morespecifically, in contrast to known embodiments of ion sources whichinclude electron emitters that include electron optics and/or an anode34, as illustrated in FIGS. 4A 4D, the embodiment of the inventionillustrated in FIGS. 4A-4D eliminates the need for an anode or otherelectron optics. Accordingly, in this embodiment of the invention, theelectron gun 12 and specifically cathode emitter 33 may be locatedcloser to the ionization volume than known ion sources, for example asillustrated in FIGS. 4A-4D forming a broad electron beam, consisting ofdispersed electrons. In addition, a static or dynamic magnetic field Bmay be employed, e.g., by means of a permanent magnet or magnetic coils(not shown), as is known in the art.

As shown in FIG. 13, the ion source 12 including the cathode 33 areimmersed in a magnetic field B in a direction as shown by the arrow inthe plane of the figure and transverse to the beam extraction directionwhich is perpendicular to the plane of the figure. It should be notedthat the magnetic field direction can be opposite to the directionshown.

In the embodiment of the invention shown in FIG. 13, the cathode 33 sitsoutside the ionization chamber volume. With such a configuration, themagnetic field traps the electrons emitted from the cathode 33 forming acolumn of electrons thus permitting operation without the electronoptics. In particular, an electron beam is emitted from the cathode 33and accelerated over the cathode-source gap, i.e the gap between thecathode 33 and the ionization chamber 5 while being trapped by theB-field. The resulting electron beam gyrates along the B-field linesmaintaining its shape until hitting the electron beam dump 11 at anopposing end of the ionization chamber. In this mode, electrons have asingle pass through the ionization chamber. Alternatively, a magneticrepeller may be installed in the beam dump location. In such anembodiment, the magnetic repeller is magnetized in the same direction asthe main source B-field. This produces a magnetic mirror from whichelectrons are reflected before hitting the beam dump. This enables mostof the electrons to pass more than once through the ionization chamberthus improving the efficiency of the source.

Referring to FIG. 13, in operation, the electron beam 32 passes throughan entrance port 45 in the ionization chamber 5 and interacts with theneutral gas within the open volume 16, defined within the ionizationchamber body. The electron beam then passes directly through a exit port36 in the ionization chamber 5 and is intercepted by the beam dump 11,which is mounted on the water-cooled mounting frame 10.

The beam dump 11 may be biased, positive or negative, relative to theelectron gun 12, i.e. cathode 33, and to the walls of the ionizationchamber 5 as well, as is known in the art. In addition, the beam dump 11may be maintained at the same or negative potential relative to theelectron gun 12 or may employ a magnetic field, such that in bothinstances the beam dump 11 acts as a repeller. Alternatively, the beamdump 11 may be electrically isolated from the ionization chamber 5 andallowed to float electrically. In this case, the beam dump 11 isself-biased by the electron beam to a voltage within a few volts of thecathode voltage, which determines the kinetic energy of the electronbeam, as is also known in the art.

The cathode 33 may be an indirectly-heated cathode (IHC), or a directlyheated filament. As is known in the art, the IHC element is held at apositive high voltage with respect to the filament, which heats the IHCby electron bombardment to a temperature sufficient for the IHC to emitan electron current.

Feed gas is introduced into the ionization chamber 5 through gas vias,for example, the vias 15 or 27 illustrated in FIG. 3, depending upon thegas to be ionized, i.e., a cluster molecule or a monatomic feedmaterial. Once inside the chamber 16, the electron beam 32 interactswith the gas to produce the ions, cluster or monatomic.

The benefits of an anode free electron source as in the presentinvention include: a reduction in the source elements, elimination ofthe anode power supply, both of which reduce complexity of the design,improved transmission of the electron beam into the ionization chamberdue to reduced electron beam space charge blow up, and elimination offailure modes associated with the anode held at a high positive voltagerelative to the cathode, namely: a reduction in thermal loading of theanode, a reduction in glitches, that is, vacuum discharges between anodeand chamber or anode and cathode; overloading of the anode voltage powersupply and a reduction in electrical shorts associated with depositedmaterial on or about the chamber entrance port 45 or the electron gun12.

An advantageous feature of the present invention is that by placing thecathode external to the ionization chamber, but still being proximate tosaid chamber, for example at a separation, d₁, measured between thecathode 33 and the chamber 5, an extraction field between the cathodeand ionization chamber can be established to efficiently extract andinject electrons into the ionization chamber, without the aid ofintermediate electron optics. In this regard, a separation distance d₁between the cathode 33 and the ionization chamber 5, substantially equalto the distance in known ion sources as illustrated in FIGS. 4A-4Dbetween the electron optics or anodes and the ionization chamber 5, forexample 3 millimeters or more, as illustrated in FIG. 16. Such aconfiguration produces sufficient electron current to produce thenecessary ionized beam of cluster gas molecules for the ion system. Moresignificantly, FIG. 22 shows that with the elimination of the electronoptics of the present invention, the extraction current versus cathodeelectron current for a monomer gas is substantially linear.

Once a plasma is formed at the surface of the cathode, the electricfield between cathode and ionization chamber will increase due to theformation of a plasma sheath. As established in the art, the plasma willsettle at a potential within several volts of the ionization chamberpotential. Since the sheath will be much narrower than the separationbetween cathode 33 and ionization chamber 5, the extraction fieldgradient increases. This allows a high extraction current whichincreases the ion beam current. Accordingly, the present invention isalso able to generate a plasma or operate in an arc discharge mode andionize suitable amounts of monomer feed gases as well as clustermolecule feed gases in a direct electron impact mode for commercial ionimplantation depending on the separation distance d between the cathode33 and the ionization chamber 5.

In accordance with a further advantageous feature of the presentinvention, is that by adjusting the separation distance d₁ of thecathode external to the ionization chamber with respect to the dimensionof the entrance port d₂, applicants can affect the ionization current ofmonomer gases. Specifically, FIG. 16 shows the ionization current of amonomer gas versus the separation distance d₁. In this regard, aseparation distance d₁ substantially less than the dimension d₂ of theentrance port 45, i.e., d₁<d₂ produces improved monomer ion beamcurrent. In this example, applicants have found that a separationdistance of about 3 mm produces the optimum bean current for monomergas.

As explained above, an advantageous feature of the present invention isobtained by placing the cathode external to the ionization chamber, butstill proximate to said chamber, for example at a separation distanced₁, an extraction field between the cathode 33 and ionization chamber 5can be established to efficiently extract and inject electrons into theionization chamber, without the aid of intermediate electron optics. Inthis case, an electron beam can be established. In some cases, it isalso advantageous to inject plasma into the ionization chamber 5 inorder to achieve a higher ion density, and hence higher ion beamcurrents. Since the external cathode 33 is necessarily in a rarifiedvacuum relative to the ionization chamber volume, conditions forcreating a plasma are not typically met unless higher electron currentscan be generated. Accordingly, in a further embodiment of the invention,this condition can be overcome by artificially raising the localpressure at the emitting surface of the cathode. A means to accomplishthis is illustrated in FIG. 15 similar to FIG. 4. A wall member orbaffle 37′ is shown attached to ionization chamber 5 and partiallysurrounds cathode 33 to increase the localized pressure proximate to thecathode 33. The baffle 37′ leaves an opening 2301 for the gas to flow tothe vacuum pump (not shown). As is known in the art, the vacuum pumpreduces the outside pressure P₀ to a level about 100 times lower thanthe pressure P within the ionization chamber, for example the pressurewithin the chamber P is on the order of 10⁻³ Torr, whereas the pressureoutside the chamber is on the order of P₀ 10⁻⁵ Torr. Insertion of baffle37′ has the effect of limiting the conductance between the volume 2302in the vicinity of the emitting surface of the cathode 33 and the pump38′, raising the pressure P_(k) of the volume 2302 to a level P_(k)>P₀.By tailoring the geometry of the baffle or wall member 37′ surroundingthe cathode 33 to adjust its conductance, the pressure at the surface ofcathode 33 can be adjusted to a given range. Raising the volume 2302pressure P_(k) will allow a plasma to form more readily, while reducingthe pressure P_(k) will restrict the formation of plasma. In this way,the onset of plasma formation which tends to characterize the transitionfrom electron impact ionization to creation of a diffuse plasmadischarge can be “tuned” for a given cathode and source geometry, whilestill maintaining the benefits of a remotely placed cathode.

Once a plasma is formed at the surface of the cathode, the electricfield between cathode 33 and the ionization chamber will increase due tothe formation of a plasma sheath. As established in the art, the plasmawill settle at a potential within several volts of the ionizationchamber potential. Since the sheath will be much narrower than theseparation between cathode 33 and ionization chamber 5 of FIG. 15, theextraction field increases. This enables higher electron currents to bedrawn from the cathode, increasing the plasma density, thus furthernarrowing the sheath. This “positive feedback” mechanism for plasmaproduction enables the onset of plasma formation to sensitively followthe local pressure at the surface of the cathode 33, making adjustmentof this pressure by appropriate design of baffle 37′ of particularutility when the higher current is required.

A magnetic field B, produced by a permanent magnet or energized coils(not shown), as disclosed above, is used in combination with thisembodiment of the ion source 1, i.e., without the electron optics oranodes. The magnetic field B can confine the primary electron beam (bothfrom the electron gun 12 and in the ionization chamber 16) to a narrowedcross-section, to reduce the spreading of the electron beam profile dueto space charge, and increasing the maximum amount of useful electroncurrent which can be injected into the ionization volume.

As discussed, these features of the embodiments of the invention asshown in FIGS. 13 through 17 have significant benefits other than thoseassociated with for example, simplicity, improvement in beamtransmission, elimination of shorting by material deposition, mentionedabove, that allows an ion source to run in multiple modes: the firstmode is electron impact and the second is a plasma discharge, similar tothe prior art arc discharge. This has the advantage of allowing theseembodiments of the ion source to produce high currents of conventionalmonomer, such as, As, P & B as well as ion beams of cluster molecules.

The fact that the ionization properties of the emitter change with thedistance of the emitter 33 from the ionization chamber 5 can beexploited by incorporating a single emitter 33 which can be deployedsuch that its position is variable between a distance d₁ and d₂ from theionization chamber. Alternatively, a single emitter 33 can be deployedwhose position d is mechanically switchable between d₁ and d₂. FIG. 17shows the latter embodiment, which is useful when switching betweenpurely electron impact ionization (for molecular ion or cluster ionformation) and arc discharge plasma formation (for production of highcurrents of monomer ions) is desired. In this embodiment, cathode 33could also extend into the ionization volume 16 of ionization chamber 5to mimic the operation of a Bernas-style immersed cathode, as indicatedin FIG. 17.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described above.

We claim:
 1. An ion source for use with an ion implant device, the ionsource comprising: a. an ionization chamber having an extractionaperture and defining an opening through which electrons may flow intothe ionization chamber; b. a gas source providing gas into theionization chamber for ionization; c. a cathode disposed outside theionization chamber and cooperating therewith; and d. a voltage supplyfor biasing the cathode with respect to the ionization chamber such thatelectrons from the cathode flow toward the ionization chamber and aredirected into the ionization chamber through the opening to ionize thegas, wherein the distance between the cathode and the ionization chamberis variable; and wherein the cathode is mechanically switchable betweena distance d₁ in which the ion source operates in arc discharge mode anda distance d₂ in which the ion source operates in a direct electronimpact mode of operation.
 2. An ion source emitter comprising: a. anionization chamber having a first aperture for receiving a source ofgas, a second aperture for permitting electrons to enter the ionizationchamber that are generated outside the ionization chamber; b. anelectron emitter for generating electrons, said electron emitterdisposed outside the ionization chamber; and c. a voltage supply forbiasing the electron emitter with respect to the ionization chamber soas to cause the electron emitter, to emit electrons into the ionizationchamber, wherein the distance between said electron emitter and saidionization chamber is adjustable; and wherein the electron emitter ismechanically switchable between a distance d₁ in which the ion sourceoperates in arc discharge mode and a distance d₂ in which the ion sourceoperates in a direct electron impact mode of operation.
 3. An ion sourcefor use with an ion implant device, the ion source comprising: anionization chamber having an electron entrance aperture; a source offeed gas in fluid communication with said ionization chamber; and anelectron emitter formed from a cathode disposed outside said ionizationchamber at a distance d₁ from said ionization chamber, wherein saiddistance d₁ is selected to cause a plasma to be generated, wherein saiddistance d₁ is adjustable; and wherein the electron emitter ismechanically switchable between a distance d₁ in which the ion sourceoperates in arc discharge mode and a distance d₂ in which the ion sourceoperates in a direct electron impact mode of operation.
 4. The ionsource as recited in claim 2, wherein said electron emitter is anodefree.
 5. The ion source as recited in claim 4, wherein said electronemitter is a cathode.
 6. The ion source as recited in claim 5, whereinsaid cathode is a directly heated cathode.
 7. The ion source as recitedin claim 5, wherein said cathode is an indirectly heated cathode.
 8. Theion source as recited in claim 4, wherein said electron emitter is afilament.
 9. The ion source as recited in claim 2, further including asource for generating a magnetic field for confining the electronsgenerated by said electron emitter.
 10. The ion source as recited inclaim 5, further including further including a baffle surrounding thecathode to raise the local pressure surrounding the cathode.
 11. The ionsource as recited in claim 2, wherein the distance that electron emitteris adjustable enables said electron emitter to be adjusted so that saidelectron emitter extends into said ionization volume.